Make send_message() with end=False generate frame when permessage-deflate is enabled

Currently, data passed by send_message() with end=False is buffered inside the
deflater object. To be consistent with send_message()'s behavior when
permessage-deflate is not enabled, make it flush given data.

This CL also adds tests to check how permessage-deflate's implementation
generates fragments.

R=yhirano@chromium.org

Review URL: https://codereview.appspot.com/95480043