Fix Image.fromarray with NumPy arrays

Image.fromarray attempts to call a method called `tobytes()` on the passed in object, but NumPy arrays don't have a `tobytes()` method, they have a `tostring()` method. (See http://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.tostring.html).

I think this was changed accidentally in a Python 3 compatibility update in which this call was confused with the `tobytes` and `frombytes` methods of Image objects.

PIL/Image.py

@@ -1944,7 +1944,7 @@ def fromarray(obj, mode=None):
 
     size = shape[1], shape[0]
     if strides is not None:
-        obj = obj.tobytes()
+        obj = obj.tostring()
 
     return frombuffer(mode, size, obj, "raw", rawmode, 0, 1)