Bug in write_ds9 for CircleSkyRegion

[cdeil]

I've run into this bug in write_ds9 for CircleSkyRegion:

>>> from astropy.coordinates import Angle, SkyCoord
>>> import regions
>>> center = SkyCoord(42, 43, unit='deg')
>>> radius = Angle(3, 'deg')
>>> region = regions.CircleSkyRegion(center, radius)
>>> regions.objects_to_ds9_string([region])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/deil/code/regions/regions/io/write_ds9.py", line 46, in objects_to_ds9_string
    x = reg.center.transform_to(frame).spherical.lon.to('deg').value[0]
TypeError: 'float' object is not subscriptable

Removing the TODO and [0] indexing here fixes the issue:
https://github.com/astropy/regions/blame/master/regions/io/write_ds9.py#L44

44        if isinstance(reg, shapes.circle.CircleSkyRegion):
45              # TODO: Why is circle.center a list of SkyCoords?
46              x = reg.center.transform_to(frame).spherical.lon.to('deg').value[0]
47              y = reg.center.transform_to(frame).spherical.lat.to('deg').value[0]

But with this change, the tests that read and write ds9 regions start failing:
https://github.com/astropy/regions/blob/master/regions/io/tests/test_ds9_language.py#L27
because after reading regions, center.shape is (1,), and not center.shape == () scalar like in my code example above.

@keflavich @joleroi - I guess the ds9 reader has to be fixed to return scalar center SkyCoord?
Does one of you have time to have a look and implement a fix?

(If I remember correctly we discussed whether regions with array parameters should be supported and PyAstro16, and I think agreed we only want to do simple scalar regions because vector regions are a nightmare to implement and of limited use, right?)

[cdeil]

Test and fix attached.

@keflavich @joleroi - Do you have time to have a look?

[cdeil]

travis-ci tests passed.
If I don't hear anything, I'll merge this tomorrow morning and try to finish up the 0.1 release.

(I know I've been saying this for two weeks, this time I really mean it.)

[keflavich]

@cdeil looks fine. Thanks for tracking this down.

[joleroi]

@cdeil I left this TODO. Thanks for fixing it!