Merge pull request matplotlib#7374 from tacaswell/fix_imsave_normalize

FIX: normalize image data in write_png fast path

Author: efiring

lib/matplotlib/image.py

@@ -524,7 +524,7 @@ def contains(self, mouseevent):
 
     def write_png(self, fname):
         """Write the image to png file with fname"""
-        im = self.to_rgba(self._A, bytes=True, norm=False)
+        im = self.to_rgba(self._A, bytes=True, norm=True)
         _png.write_png(im, fname)
 
     def set_data(self, A):