+ http: fix recent MediaType refactoring causing `*/*;q=?` mismatch, add tests

This change also add a `def withCharset(charset: HttpCharset): ContentType`
convenience method to `MediaType`.
Merge pull request #399 from spray/wip/fix-mediarange-matching

+ http: fix recent MediaType refactoring causing `*/*;q=?` mismatch, add tests