Fixed a bug that caused tests to fail.

I had one lingering issue which showed up when I ran the nose tests.
That issue is fixed and now all the original tests that were written
for the flask_mongorest package pass in the context of my changes.

My one remaining task is to write some additional tests that test
my additions.
Added a test for new POST behavior and fixed on last bug

I had one last case where I had a variable name wrong.  I also modified
the example application to introduce a uri_prefix for one of the resources
and then tested that a successful post to create that resource
returned the resource location in the Location header of the
response.  Furthermore, I made sure that the resulting URL contained
the uri_prefix associated with that resource.