delete response.content_type if assigning 204 (No Content) to response.status

When assigning the 204 status code to a response object, the `content_type` attribute is still present due to `default_content_type = 'text/html'`.

We encountered problems with this behavior in some response handling code which does not explicitly create a `HTTPNoContent` response object having `empty_body = True` but reuses an existing regular response object.

For example, http unit tests using `WebTest` croak with:
> AssertionError: Content-Type header found in a 204 response, which must not return content.
> -- webtest/lint.py, line 447, in check_content_type