Fix decimal formatting for Angle objects

I'm a little surprised this was not covered by unit tests, but here's a fix for ``to_string(decimal=True, ...)`` which ignored the ``precision`` argument previously.

Note that I've started to put more of these tests into ``test_formatting`` since I think we want to grow a robust test suite for all the different formatting options in the near future, and we might as well have a dedicated file for this.

cc @adrn @demitri @eteq @mdboom