TextualContentCompressor should compress textual Content-Type with charset

Problem:
RichHttp sets a charset by default on all textual content types, implicitly turning off all HTTP response compression enabled by TextualContentCompressor. Or for any HttpResponse created that adds a charset parameter to its Content-Type header.

Solution:
This change strips any charset parameter set on the Content-Type before doing the textual Content-Type comparison found in TextualContentCompressor, thus enabling compression for Content-Type's such as  "application/json;charset=utf-8".

Result:
Any HttpResponse with a Content-Type header that contained an approved textual content type, such as "text/html", but also included a charset parameter: "text/html;charset=utf-8", will now also have HTTP response compression enabled by default.
Can replace "charset" with just "_".