Added first few tests.
Testing <FrameObject>.representation getter and setter.
Changed preferred_representation to representation in method's docstring.
Changed the representation setter-getter tests a bit.
Working on SkyCoord initializer tests.
Added testing for unavailable attributes in a particular representation.
Tested in both spherical and cartesian representations for ICRS.
Fixed a little mistake where I'd written icrs.lat instead of icrs.ra, and so on.
Added tests for `data` object.
Checked whether it remains the same whenever representations are changed.
Started work on SkyCoord+Frame initialization tests.
Using frame ICRS.
Merge branch 'coord-pref-tests' of github.com:VaticanCameos/astropy into coord-pref-tests