Conditionally set X-Frame-Options header when proxying to S3
Conditionally set X-Frame-Options header when proxying to S3

This mirrors the conditional in proxy-vhost.conf and ensures that we
only set the `X-Frame-Options` header when proxying to S3 if it's also
set when servings assets from the file system.

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