Change the way middleware handles POST requests

[tim-mccurrach]

At the moment, in the case of a POST request, the middleware only looks in request.POST for the JWT_QUERYSTRING_ARG key. This works well for submitting forms, however Django request.POST is empty when you submit a request with headers: { "Content-Type": "application/json" }. It only fills up when the content-type is normal HTML form submission values (either multipart/form-data or application/x-www-form-urlencoded). In other cases it is necessary to check in request.body.

It might also be useful to look for the JWT_QUERYSTRING_ARG key in request.GET for POST requests as well as GET requests. This will mean the token can be added on the end of a url as a query-string and might simplify the javascript layer in many cases.