Set contentType to JSON when sending session models

[takluyver]

Closes ipython/ipython#8416

Should be backported to 3.2 as well, since we seem to be planning for another 3.x release.

[Carreau]

+1

[rgbkrk]

Yep, that makes sense.

[rgbkrk]

The error is name change related, only 5 minutes before this PR was opened.

======================================================================
ERROR: test_list_formats (jupyter_notebook.services.nbconvert.tests.test_nbconvert_api.APITest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/jupyter/jupyter_notebook/jupyter_notebook/services/nbconvert/tests/test_nbconvert_api.py", line 27, in test_list_formats
    formats = self.nbconvert_api.list_formats().json()
  File "/home/travis/build/jupyter/jupyter_notebook/jupyter_notebook/services/nbconvert/tests/test_nbconvert_api.py", line 20, in list_formats
    return self._req('GET', '')
  File "/home/travis/build/jupyter/jupyter_notebook/jupyter_notebook/services/nbconvert/tests/test_nbconvert_api.py", line 16, in _req
    response.raise_for_status()
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/requests/models.py", line 851, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
HTTPError: 500 Server Error: Internal Server Error
-------------------- >> begin captured logging << --------------------
NotebookApp: INFO: Shutting down kernels
root: DEBUG: Installing kernelspec in /tmp/tmpkAKO0V/.local/share/jupyter/kernels/python2
NotebookApp: INFO: Serving notebooks from local directory: /tmp/tmpegAI0t
NotebookApp: INFO: 0 active kernels 
NotebookApp: INFO: The IPython Notebook is running at: http://localhost:12341/
NotebookApp: INFO: Use Control-C to stop this server and shut down all kernels (twice to skip confirmation).
requests.packages.urllib3.connectionpool: INFO: Starting new HTTP connection (1): localhost
requests.packages.urllib3.connectionpool: DEBUG: "GET /api/contents HTTP/1.1" 200 214
requests.packages.urllib3.connectionpool: INFO: Starting new HTTP connection (1): localhost
NotebookApp: WARNING: Could not import nbconvert: No module named jupyter_nbconvert.exporters.export
requests.packages.urllib3.connectionpool: DEBUG: "GET /api/nbconvert HTTP/1.1" 500 109
NotebookApp: ERROR: {
  "Host": "localhost:12341", 
  "User-Agent": "python-requests/2.7.0 CPython/2.7.9 Linux/3.13.0-40-generic", 
  "Connection": "keep-alive", 
  "Accept": "*/*", 
  "Accept-Encoding": "gzip, deflate"
}
NotebookApp: ERROR: 500 GET /api/nbconvert (::1) 2.00ms referer=None

I think the namespace changed for jupyter_nbconvert to nbconvert in jupyter/nbconvert#18, only 30 minutes ago.

[rgbkrk]

With new commit comes new merge commit testing!

[takluyver]

It looks like #71 should have fixed all the references to nbconvert.

[takluyver]

With new testing comes success.