Allow for extra media-type parameters in content-type header

Stinger limits POST requests to only accept those with content type [1] `application/x-www-form-urlencoded` but does not allow for media-type [2] values that contain additional parameters.

We fix this by only matching against the prefix of the header value.

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[1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.17
[2] http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7