Added tests for Content-Type with charset.

Added some tests for Content-Type headers that also
specify a charset.
Added an additional testcase.
Fix for Content-Type parameter issue.

Fix for https://github.com/jwagenleitner/groovy-wslite/issues/74.
Use existing utility class to parse header.
Merge pull request #76 from neil-greenwood/master

Parsing of Content-Type containing 'charset' parameter.