Toolbar can break JSON POSTs

As soon as I enable pyramid_debugtoolbar a JSON POST to Pyramid breaks for clients which forget to send a `Content-Type` header. This is caused by Pylons/webob#149, but I wonder if the debug toolbar (specifically `RequestVarsDebugPanel`) should even try access request.POST if request.content_type is not `application/x-www-form-urlencoded` or `multipart/form-data`.