mediawiki.Uri: Optional url use default location.
This patch does three things:
1. If mw.Uri doesn't get any arguments, it will return the default
URI. This is a good default, because it's something that will be
used often, and the URI object that refers to the default location
already exists locally, we're just using it to return a clone of
that.
2. Fixes a strange logic error that prevents #1 from taking effect
properly. The last clause in the 'if' statement on line 120
meant that if uri is undefined, the if block would execute. I
doubt that was intended, but it happened, and now it's fixed.
There may be some error handling in mw.Uri.parse that could be
removed since the URI passed in will almost never be undefined.
3. Adds tests for the new constructor values.
Change-Id: I9254b89bba41572c6dce72d2100d7085c37fb2c3