Fix test for stretch

- fix #159
- fixes test for "stretch"; use method API to fetch the distribution
  object rather than accessing instance variable

Author: ejeschke

astrowidgets/core.py

@@ -814,7 +814,7 @@ def stretch(self):
         """
         The image stretching algorithm in use.
         """
-        return self._viewer.rgbmap.dist
+        return self._viewer.rgbmap.get_dist()
 
     # TODO: Possible to use astropy.visualization directly?
     @stretch.setter