Content-type header check now case insensitive

Thanks to @ccpmark for finding this bug and @mcos for finding the solution.

As stated in [RFC1341](http://www.w3.org/Protocols/rfc1341/4_Content-Type.html):

> The type, subtype, and parameter names are not case sensitive. For example, TEXT, Text, and TeXt are all equivalent.

So `application/JSON` and `ApPlICaTioN/JsOn` etc should be treated as the same during the request.