Issue title: Galactic Coordinates recently broken
 Issue body: ```
In [1]: from astropy import coordinates as coords

In [2]: from astropy import units as u

In [3]: c = coords.GalacticCoordinates(l=-67.02084, b=-29.75447, unit=(u.deg, u.deg))

In [4]: c
Out[4]: <GalacticCoordinates l=292.97916 deg, b=-29.75447 deg>

In [5]: c.lonangle
Out[5]: <Longitude 292d58m44.97600s>

In [6]: c.latangle
Out[6]: <Latitude -29d45m16.09200s>

In [7]: import astropy

In [8]: astropy.__version__
Out[8]: '0.3.dev5163'
```

 Comments: 
 Comment 0: Huh, how was this not caught by the unit tests? Our test coverage of astropy.coordinates needs a little improvement...

 Comment 1: Wonder why https://github.com/astropy/astropy/blob/master/astropy/coordinates/tests/accuracy/test_galactic_fk4.py didn't catch it - don't have time to investigate right now, but would be worth checking.

 Comment 2: Agreed, we need better test coverage.  

I tried investigating the problem, but blame on `builtin_systems.py` didn't reveal anything obvious.  It may have been a subtle change in some other module, or an unexpected consequence of b37cbb6b, but I really don't know.

 Comment 3: If you're in the mood, you could try a bisect (but no worries if you don't have time)

 Comment 4: @keflavich - Can you describe the breakage?  

> Galactic coordinates are being automatically converted to RA/Dec. This breaks astroquery completely.

I'm a little surprised that astroquery has this sensitivity, since the previous behavior (prior to #1310) was that the lat/lon angles for Galactic coordinates were Angle objects, and now they are Latitude/Longitude objects, which are just thin subclasses of Angle.

Note that somewhere along the line Galactic longitude starting being in the range 0-360 and instead of -180 to 180.  I think that happened before #1310 but I'm not totally sure of that.  Is that the problem?

 Comment 5: Ah, 292.97916 is just the same as the initial l but wrapped - makes sense! I agree this shouldn't break Astroquery though.

 Comment 6: Oh, you're entirely right.  Sorry, I was quite rushed when writing this issue.  The problem is actually just an assertion that input lonangle = output lonangle, which is the wrong check to be performing.  I should have realized that lat == lat but lon != lon.

{'source': 'GithubIssues', 'repo_name_with_owner': 'astropy/astropy', 'issue_id': 18837728, 'repo_stars': 3557.0}