displaypub b64 encoding is non-destructive

`_encode_binary` helper fund encoded in-place, which is destructive
if the dict is ever re-used.

also prevents double-encoding, by only encoding bytes objects, since unicode objects are necessarily already encoded.

IPython/zmq/displayhook.py

@@ -31,12 +31,15 b' class ZMQDisplayHook(object):'
 
 
 def _encode_binary(format_dict):
+    encoded = format_dict.copy()
     pngdata = format_dict.get('image/png')
-    if pngdata is not None:
-        format_dict['image/png'] = encodestring(pngdata).decode('ascii')
+    if isinstance(pngdata, bytes):
+        encoded['image/png'] = encodestring(pngdata).decode('ascii')
     jpegdata = format_dict.get('image/jpeg')
-    if jpegdata is not None:
-        format_dict['image/jpeg'] = encodestring(jpegdata).decode('ascii')
+    if isinstance(jpegdata, bytes):
+        encoded['image/jpeg'] = encodestring(jpegdata).decode('ascii')
+    
+    return encoded
 
 
 class ZMQShellDisplayHook(DisplayHook):
@@ -61,8 +64,7 b' class ZMQShellDisplayHook(DisplayHook):'
         self.msg['content']['execution_count'] = self.prompt_count
 
     def write_format_data(self, format_dict):
-        _encode_binary(format_dict)
-        self.msg['content']['data'] = format_dict
+        self.msg['content']['data'] = _encode_binary(format_dict)
 
     def finish_displayhook(self):
         """Finish up all displayhook activities."""

IPython/zmq/zmqshell.py

@@ -77,8 +77,7 b' class ZMQDisplayPublisher(DisplayPublisher):'
         self._validate_data(source, data, metadata)
         content = {}
         content['source'] = source
-        _encode_binary(data)
-        content['data'] = data
+        content['data'] = _encode_binary(data)
         content['metadata'] = metadata
         self.session.send(
             self.pub_socket, u'display_data', json_clean(content),