Enable this example service to work with django-rest-framework-jwt

[matthewhegarty]

I made some minor changes to get this example to work with django-rest-framework-jwt (without affecting the core example).

Firstly, calls to DRF failed because of HTTP 415 (unsupported media type) (SO). This was corrected by adding 'Content-Type': 'application/json' header.

Secondly, DRF returns HTTP 400 to indicate authentication failure (docs. This error needs to be caught to indicate auth failure.

Changes:

1. Added json content-type header
2. Enabled http.post call to handle error
3. Enabled login component to catch errors and disallow authentication