Include the content in `payload_json` for file uploads

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).

src/Discord.Net.Rest/API/Rest/UploadFileParams.cs

@@ -30,28 +30,24 @@ namespace Discord.API.Rest
         {
             var d = new Dictionary<string, object>();
             d["file"] = new MultipartFile(File, Filename.GetValueOrDefault("unknown.dat"));
+
+            var payload = new Dictionary<string, object>();
             if (Content.IsSpecified)
-                d["content"] = Content.Value;
+                payload["content"] = Content.Value;
             if (IsTTS.IsSpecified)
-                d["tts"] = IsTTS.Value.ToString();
+                payload["tts"] = IsTTS.Value.ToString();
             if (Nonce.IsSpecified)
-                d["nonce"] = Nonce.Value;
+                payload["nonce"] = Nonce.Value;
             if (Embed.IsSpecified)
-            {
-                var payload = new StringBuilder();
-                using (var text = new StringWriter(payload))
-                using (var writer = new JsonTextWriter(text))
-                {
-                    var map = new Dictionary<string, object>()
-                    {
-                        ["embed"] = Embed.Value,
-                    };
-
-                    _serializer.Serialize(writer, map);
-                }
-                d["payload_json"] = payload.ToString();
-
-            }
+                payload["embed"] = Embed.Value;
+
+            var json = new StringBuilder();
+            using (var text = new StringWriter(json))
+            using (var writer = new JsonTextWriter(text))
+                _serializer.Serialize(writer, payload);
+
+            d["payload_json"] = json.ToString();
+
             return d;
         }
     }

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -180,7 +180,7 @@ namespace Discord.Rest
         public static async Task<RestUserMessage> SendFileAsync(IMessageChannel channel, BaseDiscordClient client,
             Stream stream, string filename, string text, bool isTTS, Embed embed, RequestOptions options)
         {
-            var args = new UploadFileParams(stream) { Filename = filename, Content = text, IsTTS = isTTS, Embed = embed?.ToModel() };
+            var args = new UploadFileParams(stream) { Filename = filename, Content = text, IsTTS = isTTS, Embed = embed != null ? embed.ToModel() : Optional<API.Embed>.Unspecified };
             var model = await client.ApiClient.UploadFileAsync(channel.Id, args, options).ConfigureAwait(false);
             return RestUserMessage.Create(client, channel, client.CurrentUser, model);
         }