The Content-Type header must be ignored when the Transfer
Encoding is set to "chunked" because the length is unknown

Go sets this to -1 and we pass that onto the user:
https://golang.org/src/net/http/transfer.go



The value of Content_Length is currently set to -1 in this
scenario, however it caused some confusion for at least one
user in issue: #1422.

The Http_Transfer_Encoding value was tested by running the
watchdog on Linux with "env" as the fprocess and an extra
header to "curl"

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>