Do not set 'Content-Type' header for all backend if http code does not allow message-body

[inkhey]

Find a solution to not set 'Content-Type' header if unallowed for all backend. https://github.com/algoo/hapic/commit/76d5f8c396583a53bd141b470216a4345a8c75c8 is pyramid specific solution for this. It should be replaced by something not pyramid specific.