Parse content-type header in a spec complicant way

[anotherdoesnm]

The Eris library (and potentially other related libraries) enforces strict checks on the application/json content type when processing JSON responses (this behavior is likely influenced by Discord's implementation). However, the Content-Type header can include additional parameters, such as encoding, separated by a semicolon, and yhis flexibility causes issues when using Eris with Spacebar, an open-source reimplementation of the Discord API.