mediawiki.Uri: Fix host example and change foo.com to example.com

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