Add unregister_template_handler to prevent leaks.

A few places in the test codebase had `ActionView::Template.register_template_handler`, but the handlers were not removed after the tests.

This patch adds a new method `unregister_template_handler` to `ActionView::Template::Handlers`, and makes sure related tests are cleaned up properly.

cc @senny
It still looks like you're testing render and not "unregistering the extension". The test case should say how render behaves in that case. For example:

`test_render_does_not_use_unregistered_extension_and_template_handler`