Ability to forward some headers

[JeffersonBledsoe]

This PR is a replication of the functionality in collective.easyform, allowing the following headers to be forwarded:

• HTTP_X_FORWARDED_FOR
• HTTP_X_FORWARDED_PORT
• REMOTE_ADDR
• PATH_INFO
• HTTP_USER_AGENT
• HTTP_REFERER

[JeffersonBledsoe]

@cekk Done!