Bug fix for AnyEvent::WebSocket::Connection not receiving "" or "0"

AnyEvent::WebSocket::Connection could not receive the message of "" (empty string) or "0" (character of zero).
This patch includes the test and fix for it.

The fix, however, made mojo.t and ssl.t fail, because now empty control frames are passed to on_each_messages() callbacks.
I modified Connection.pm so that it would pass only data frames to the callbacks.