don't mutate a user's filename

if a user specifies ‘test.png’ as their filename, then don’t fuck with
that! now, we were changing that to `’test.png.png’` if they also
specified `format=‘png’`. sure, now they could set their filename
`test.png` and their format `jpeg`, but that is their fault.
this fixes the test, too.

Author: chriddyp

plotly/plotly/plotly.py

@@ -653,9 +653,7 @@ def save_as(cls, figure_or_data, filename, format=None, width=None,
         elif ext and not format:
             format = ext[1:]
         elif not ext and format:
-            filename += '.'+format
-        else:
-            filename += '.'+format
+            filename += '.' + format
 
         img = cls.get(figure_or_data, format, width, height)