This resolves #987

Previous behavior was that even if `null` was passed for an embed in
UploadFileAsync, the Embed property on UploadFileArgs was still
specified - this meant we were always sending a payload_json.

If a payload_json is specified, it seems like Discord will only read
from the payload_json, and will ignore properties set outside of it.

To prevent unnecessary code duplication, this commit always specifies
parameters in the payload_json, and also will only include the embed if
one was actually specified with real data (not null).

commit a8bafb90cd
Merge: f38dd4c4 7e04285e
Author: WamWooWam <wamwoowam@gmail.com>
Date:   Mon Mar 12 08:05:52 2018 +0000

    Merge branch 'dev' of https://github.com/WamWooWam/Discord.Net into dev

commit f38dd4c421
Author: WamWooWam <wamwoowam@gmail.com>
Date:   Mon Mar 12 08:05:49 2018 +0000

    Cleaned up & fixed code style.

commit 7e04285e5d
Author: Thomas May <wamwoowam@gmail.com>
Date:   Sun Mar 11 14:11:28 2018 +0000

    Revert changes to DefaultRestClient

    Didn't actually need to change this, whoops.

commit 3f5b2c8ef1
Author: WamWooWam <wamwoowam@gmail.com>
Date:   Sat Mar 10 19:30:44 2018 +0000

    Enabled embeds alongside uploaded files.

    God damn Discord is a mess.

Co-authored-by: WamWooWam <wamwoowam@gmail.com>

* Fix #854 Added ViewChannel enum and property to channel permissions

* replaced usages of ChannelPermission#ReadMessages with ViewChannel

* rename parameter of ChannelPermissions constructor

* made OverwritePermissions#ReadMessages obsolete, use ViewChannel instead

* Fix #854 Added ViewChannel enum and property to channel permissions

replaced usages of ChannelPermission#ReadMessages with ViewChannel

rename parameter of ChannelPermissions constructor

made OverwritePermissions#ReadMessages obsolete, use ViewChannel instead

* renamed readMessages parameter in ChannelPermissions constructor and Modify

* fixed channel permission tests to use ChannelPermission enum instead of GuildPermission enum

* replaced usages of readmessages in channel permission tests

* resolve build warnings for permission tests

* Fix flawed bulk message deletion

https://github.com/RogueException/Discord.Net/issues/871, consider changing DeleteMessagesParams.MessageIds type to I(Readonly)List<ulong> or IEnumerable<ulong> to avoid unnecessary copying (batch.ToArray())

* Update code formatting

commit a85c5814a7
Author: Christopher F <computerizedtaco@gmail.com>
Date:   Sat Jan 6 22:25:48 2018 -0500

    Code cleanup

commit 4b243fd3dd
Author: Christopher F <computerizedtaco@gmail.com>
Date:   Sat Jan 6 22:08:28 2018 -0500

    Add support for channel categories (#907)

    commit 41ed9106f2
    Author: mrspits4ever <spits.lucas@gmail.com>
    Date:   Thu Dec 14 20:02:57 2017 +0100

        removed mentioning support for RestCategoryChannel, added channels property to SocketCategoryChannel

    commit 71142c3108
    Merge: 4589d731 678a7238
    Author: mrspits4ever <spits.lucas@gmail.com>
    Date:   Wed Dec 13 21:17:53 2017 +0100

        Merge branch 'dev' of https://github.com/RogueException/Discord.Net into feature/channel-categories

    commit 4589d73187
    Author: mrspits4ever <spits.lucas@gmail.com>
    Date:   Wed Dec 13 21:17:46 2017 +0100

        adressed requested changes

    commit d59b038efa
    Author: pegasy <pegasy@users.noreply.github.com>
    Date:   Mon Sep 25 18:53:23 2017 +0200

        Renamed classes / properties / methods to use CategoryChannel instead of ChannelCategory to be consistant with how text / voice channels are named.

    commit 5c4777dc8c
    Author: pegasy <pegasy@users.noreply.github.com>
    Date:   Sun Sep 24 19:08:25 2017 +0200

        removed Guild from class name for ChannelCategory
        Renamed all properties to use Category instead of Parent
        Throw exception on GetUsers / GetInvites etc for categories

    commit e18bd8c799
    Author: pegasy <pegasy@users.noreply.github.com>
    Date:   Sun Sep 24 15:49:51 2017 +0200

        Add support for channel categories (as its own channel type)

* Add Webhook API models, REST implementation, and Socket bridges.

* Remove token overrides from REST.

Leaving that as a Webhook package only feature.

* Add Webhook API models, REST implementation, and Socket bridges.

* Remove token overrides from REST.

Leaving that as a Webhook package only feature.

* Webhook core implementation.

* Webhook REST implementation.

* Webhook client implementation.

* Add channel bucket id.

IChannel.IsNsfw will now return false when being used on any channel
that is not an ITextChannel. When being used on an ITextChannel, this
will now account for the API flag, and fall back to the channel name.

(this is gross design, thanks discord)

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

* Update ChannelHelper.cs

* typo

Resolves #469

this was a big one

I'm not entirely sure this is the proper way to fix this. However, it seems like
SendFileAsync returns the Task<RestUserMessage>, which would exit the using
block, disposing of the FileStream potentially before the Task is finished.

Changed to await the result of the task before exiting the using block.