Angles.to_string fix to get string outputs

This closes #2980 and adds a regression test for it.

The fix is really straightforward, but @mdboom, can you clarify if there's some particular reason why np.object was originally specified as the ``otype`` that I'm missing?

cc @cdeil @taldcroft 
Probably should be `>= 1.6` in case a security fix called 1.5.2 is ever released.