mediawiki.Uri: Fix host example and change foo.com to example.com
* The example changed host from foo.com to www.foo.com, but
the .toString() comment showing the expected value was wrong
as it implied setting .host doesn't work, but it does.
* Update examples and tests to use example.org or example.com
instead of foo, test, or google.com etc.
Follows-up
a94417720248dd3 which accidentally broke the example,
when fixing the .setAttr() -> .attr() mistake.
Change-Id: Ifeaf713a8f1db8758b01aa84061b49316ca5d577