Allow parameters, such as content encoding, in content_type.

For example, if we define :json content type as "application/json; charset=utf-8"
and client do payload(POST/PUT) request with "application/json" type then response should be successful.

Ignore content type params in mime_types method.
Merge pull request #508 from dm1try/content_type_with_params

Allow parameters, such as content encoding, in content_type.