Add map to calculate x-forwarded-host header
Add map to calculate x-forwarded-host header

This map uses the existing x-forwarded-host header if present, if not
defaults to the original host header.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet