* Overwrite proxy headers based on directive
Headers of the request sent by the proxy upstream can now be modified in
the following way:
Prefix header with `+`: Header will be added if it doesn't exist
otherwise, the values will be merge
Prefix header with `-': Header will be removed
No prefix: Header will be replaced with given value
* Add missing formating directive reported by go vet
* Overwrite up/down stream proxy headers
Add Up/DownStreamHeaders to UpstreamHost
Split `proxy_header` option in `proxy` directive into `header_upstream`
and `header_downstream`. By splitting into two, it makes it clear in
what direction the given headers must be applied.
`proxy_header` can still be used (to maintain backward compatability)
but its assumed to be `header_upstream`
Response headers received by the reverse proxy from the upstream host
are updated according the `header_downstream` rules.
The update occurs through a func given to the reverse proxy, which is
applied once a response is received.
Headers (for upstream and downstream) can now be modified in
the following way:
Prefix header with `+`: Header will be added if it doesn't exist
otherwise, the values will be merge
Prefix header with `-': Header will be removed
No prefix: Header will be replaced with given value
Updated branch with changes from master
* minor refactor to make intent clearer
* Make Up/Down stream headers naming consistent
* Fix error descriptions to be more clear
* Fix lint issue
