Allow DELETE to send content in tests

Werkzeug (and in turn, Flask) doesn't support sending `data` content with a `DELETE` request.

I just ran into this issue in my own tests, and found someone on Stack Overflow with a similar question: http://stackoverflow.com/q/24073488/1309238

Following the debugger through the request, it looks like the issue is here in this pull request, where the `_get_content_type` returns `None` for DELETE requests.  As I mention in my answer to that Stack Overflow question, this is inconsistent with how HTTP libraries (like curl) can be used to send DELETE requests with content.

I wasn't able to successfully run the tests on my local machine, so I'm not sure if this method is exercised or checked by anything.  I'm happy to add a test that covers this, if it's needed to merge into the repo.