Only lower-case ContentType's values if the name is 'charset'.

BUG=https://code.google.com/p/dart/issues/detail?id=21385
R=lrn@google.com, sgjesse@google.com

Review URL: https://codereview.chromium.org//694293002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41456 260f80e4-7a28-3924-810f-c04153c831b5