Support charset parameter of Content-Type header

According to the [XMLHttpRequest spec](http://www.w3.org/TR/XMLHttpRequest/), the following is a valid `Content-Type` header:

```
Content-Type: text/plain; charset=utf-8
```

Firefox sends this when making AJAX requests, so I'm trying to write a test case using superagent for my response handler to make sure it handles accordingly. However, superagent uses the value of the content type header as the lookup on a key of serialisers.

This pull request parses out the first part of the header to use as the lookup key. Hope it's ok. Let me know!