Copy headers using common method
Copy headers using common method

This change moves to copy headers via copyHeaders which is used
in other OpenFaaS proxying code.

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

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