Raise BadRequest rather than AssertionError in MongoEngineResource.dispatch()

[dcooper]

If an API client sends an invalid request to the API server, the server should return a 4xx response rather than a 5xx response.

The dispatch() method on MongoEngineResource raises AssertionError when testing the request method. If dispatch() was to raise tastypie_exceptions.BadRequest rather than AssertionError, then tastypie will return an HTTP 400 to the client rather than an HTTP 500.

In the 0.4.2 version of tastypie_mongoengine, the AssertionError exceptions are raised in the following places:
tastypie_mongoengine/resources.py:373
tastypie_mongoengine/resources.py:376

[mitar]

Are you sure this should not be checked already in Tastypie? Here is assert just to assure we are using Tastypie API properly.

But if you want, you can make a pull request with this change and please add tests which send invalid requests and fail.