Django client's get_data_from_request method did not correctly assign request.POST 

This PR is to fix Raven's django client assigning string ``<unavailable>`` on ``result['sentry.interfaces.Http']['data']`` on method ``get_data_from_request``, even when we have correct POST variables in the request.

Also in this PR, I added test case for this issue


Before:
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![selection_143](https://f.cloud.github.com/assets/1240672/1113109/b9588832-19f3-11e3-8d3a-4d8627000344.png)



Result after this change:
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![selection_142](https://f.cloud.github.com/assets/1240672/1113117/dd7eda86-19f3-11e3-9e67-eec3bfbf44e0.png)