Content type is not being split against ";"
According to RFC Content-Type header field may contain parameters: http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7
For instance, Firefox XHR request adds charset parameter: application/json; charset=UTF-8
Attached patch splits Content-Type against ";" and takes only first item as a Content-Type. In addition, it fixes loader_for_type function which now can do strict checking against mime string.