Frame: remove fin() accessor from documentation and test. If a message is fragmented, the return value of fin() is meaningless anyway. Plus, probably current implementation does not handle fin flags if control frames are injected in non-control fragments. If we want fin() accessor, the test for that case is necessary.
Frame: add documentation of "masked" field in new() method.
Frame: add documentation of "version" field in new() method.
Frame: documentation: add remarks about destructiveness of append() method. Sometimes it's useful, the other times it's confusing.
Frame: add test for destructiveness of append() method.