Match Content-Type header

The Content-Type header is no longer filtered when:
 - Matching
 - Tracking request data

The [rack specification](https://github.com/rack/rack/blob/master/SPEC) identifies that the Content-Type header should not be prefixed with HTTP_ which is why it was previously being ignored as not a request header. 

This fixes and closes #13 and is release in v3.0.7