Permit a dangling semicolon in media type parsing.

I found this in-the-wild with Crawler.
Merge pull request #686 from square/jwilson_0413_dangling_semicolon

Permit a dangling semicolon in media type parsing.
1.5.4 changelog.

(cherry picked from commit b77be40c67c398c146f3269717393a2aad70e43a)
Merge branch 'square_master'