Fix infinite recursion in units with ndarray subclasses.

This was partially addressed for masked arrays in matplotlib#2290. However,
this does not work for all ndarray sub-classes, like the quantities
package. The fix here is to make sure ravel is actually succeeding
in changing the shape of the object. If it doesn't just return
what we've got.

Author: dopplershift

lib/matplotlib/units.py

@@ -149,7 +149,13 @@ def get_converter(self, x):
                     return converter
             except AttributeError:
                 # not a masked_array
-                converter = self.get_converter(xravel[0])
+                # Make sure we don't recurse forever -- it's possible for
+                # ndarray subclasses to continue to return subclasses and
+                # not ever return a non-subclass for a single element.
+                next_item = xravel[0]
+                if (not isinstance(next_item, np.ndarray) or
+                    next_item.shape != x.shape):
+                    converter = self.get_converter(next_item)
                 return converter
 
         if converter is None and iterable(x):