Do not strip parameters from the Content-Type header.

According the the Ring spec, the implementation was correct. But, the
Ring spec does not conform to the the HTTP RFC [1]. A bug has been
filed [2]. The parameters in the Content-Type header are needed, for example
when using the multipart-params middleware.

[1] http://www.w3.org/Protocols/rfc1341/4_Content-Type.html
[2] https://github.com/ring-clojure/ring/issues/103