Fix settings-related test-order bug.

It turns out that although database fixtures are reset between test
functions, the global settings object is persisted, and therefore
settings changed during one test can affect others. When running all of
the unit tests at once, it so happened that due to namings / internal
Python decisions on which tests to run first, two tests of authorization
confirmation were being run with `DJOAUTH2_SSL_ONLY = False`. This hid
the fact that the tests were making POST requests without using SSL when
required. Running the test command individually ( `./runtests.py
djoauth2.TestMakeAuthorizationEndpointHelper`) led to the global
`DJOAUTH2_SSL_ONLY` settings NOT being changed from the default
(`True`), causing the tests to (rightfully) fail.

This commit implements a `setUp` method on all `DJOAuth2TestCase`s that
restores the default `djoauth2`-related settings before running the test
case.

This commit implements a helper for confirming authorization request
that respects the current state of the `DJOAUTH2_SSL_ONLY` setting, just
like the other request helpers.

This commit changes the default server URL from 'testserver' to
'locu.com', so that useful URLs can be parsed from 'Location'
attributes. It could be any real domain, but hey, we like Locu.