don't 'restore_bytes' in from_JSON

it makes no sense to turn base64-encoded unicode strings into base64-encoded byte strings.  I can't think why we do this, but we should be very careful about testing before merging this fix.

fixes the issue tested in  #4036.

IPython/nbformat/v3/nbbase.py

@@ -69,9 +69,9 b' def new_output(output_type=None, output_text=None, output_png=None,'
         if output_text is not None:
             output.text = unicode(output_text)
         if output_png is not None:
-            output.png = bytes(output_png)
+            output.png = unicode(output_png)
         if output_jpeg is not None:
-            output.jpeg = bytes(output_jpeg)
+            output.jpeg = unicode(output_jpeg)
         if output_html is not None:
             output.html = unicode(output_html)
         if output_svg is not None:

IPython/nbformat/v3/nbjson.py

@@ -46,7 +46,7 b' class JSONReader(NotebookReader):'
         return nb
 
     def to_notebook(self, d, **kwargs):
-        return restore_bytes(rejoin_lines(from_dict(d)))
+        return rejoin_lines(from_dict(d))
 
 
 class JSONWriter(NotebookWriter):