This prevents a 400 when sending a malformed color to Discord.

**This is not a breaking change**.

This change begins to implement #379, where IUserMessage.ModifyAsync will now accept a `Discord.ModifyMessageParams` func over a `Discord.API.Rest.ModifyMessageParams` func.

This resolves #224

Make header dictionary ignore case

Resolves an issue where the ratelimiter would panic.

Access "Date" in response header safely with TryGetValue. Hopefully nothing bad happens when lag is 0

In cases where Discord sent a value of `id=0`, this would throw an invalid-cast, where 0u64 cannot be cast to string.

this is all really broken