mediawiki.Uri: Optional url use default location.

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