XmlWriterWrap should be abstract and have final methods

[victornoel]

I see XmlWriterWrap was introduced in #109.

I believe that envelopes should be abstract and have all their methods be final : that would allow for removing the DesignForExtensionCheck suppression from checkstyle. It exists for good reasons, it is dangerous to use inheritance, but it is way safer when one can't override inherited behaviour.

Also it should be abstract because as a good rule, classes should be either final or abstract to avoid confusing situation with inheritance.

[0crat]

@g4s8/z please, pay attention to this issue

[victornoel]

@g4s8 also I'm in favour of naming it XMLStreamWriterEnvelope to reflect both the interface it envelopes and the fact it is an envelope.