WireCloud proxy adds a Content-Type header to the requests

[aarranz]

Seems that Django or the WSGI is adding a default value for the Content_Type meta value so the proxy is not able to detect if the request provides a Content-Type header or not.

This has been detected on a request to the v2 API of the context broker:

{
    "error": "BadRequest",
    "description": "Orion accepts no payload for GET/DELETE requests. HTTP header Content-Type is thus forbidden"
}