Add Http_Transfer_Encoding
Add Http_Transfer_Encoding

This affects serializing and streaming modes and lets the caller
know what the encoding mode is when that is set. For instance if
the value is "chunked", the Http_Content_Length must be ignored.

Closes: #89

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <5f7133baa0f5c7ca63ff11e11f2e2b0d2cf077c8@gmail.com>

openfaas-incubator/of-watchdog,openfaas-incubator/of-watchdog