update payload source

should be simple key, not Python (or worse, IPython) specific long key.

I could have sworn I did this long ago,
but apparently the source key was only updated in display_data,
not execute_reply.
mention payloads for non-Python kernels

they should be ignored.
Merge pull request #3844 from minrk/payload

update payload source

should be simple key, not Python- (or worse, IPython-) specific long key.

I could have sworn I did this long ago, but apparently the source key was only updated in display_data, not execute_reply.