Add tests for permessage-deflate in test_msgutil.py.
Add tests for permessage-deflate in test_msgutil.py.

Since the existing tests for permessage-compress covers most of code path
used for permessage-deflate, I don't add the same ones, but just these simple
two cases.

Release note: n/a (test only change)

Review URL: https://codereview.appspot.com/8541048

git-svn-id: d23f85197b9dafcb1b1682eb9d0eda70517bdc52@759 4ff78f4a-9131-11de-b045-6380ec9940d4

bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src,bpsinc-native/src_third_party_pywebsocket_src