Extract session buffers as memoryviews

A bug in pyzmq forces us to have a workaround, which can be deleted when the pyzmq dependency is upgraded to fix pyzmq issue  #646.

Thanks to @minrk for helping work on the code and also for the bugfix in pyzmq.

IPython/kernel/zmq/session.py

@@ -842,8 +842,11 b' class Session(Configurable):'
             message['content'] = self.unpack(msg_list[4])
         else:
             message['content'] = msg_list[4]
-
-        message['buffers'] = msg_list[5:]
+        buffers = [memoryview(b) for b in msg_list[5:]]
+        if buffers and buffers[0].shape is None:
+            # force copy to workaround pyzmq #646
+            buffers = [memoryview(b.bytes) for b in msg_list[5:]]
+        message['buffers'] = buffers
         # adapt to the current version
         return adapt(message)