cwtags #4502759 UStringIO monkeypatch breaks assertions in write() [validation pending]

UStringIO.write() has assertions to make sure that only Unicode strings are given to it. The ways cwtags monkeypatches UStringIO has many drawbacks:

• it bypasses completely the assertion in place, allowing anything to be given to write(),
• the monkeypatch of getvalue() worsens this by implicitely calling unicode() on the content,
• the current implementation depends on the knowledge of UStringIO's implementation (namely, the fact that UStringIO inherits from list)

A better approach would be to call the original method from the monkeypatch (much like a super call would) to get all the benefits from the original method, while adding new handling for tag objects.