message: Update https_support.html
  Make note of `X-Forwarded-Proto` and https termination potentially
  breaking fetches on behalf of html due to an arising protocol mismatch.
  (And suggest explicitly alllowing the domain to resolve).
diff: @@ -284,7 +284,14 @@ under the License.
  URLs and thus avoid mixed content warnings. Note, that you should only
  enable this option if you are behind a load-balancer that will set this
  header, otherwise your users will be able to set the protocol
  - PageSpeed uses to interpret the request.
  + PageSpeed uses to interpret the request. Also, note that by default
  + PageSpeed will loop back to the inbound ip/port for fetching resources
  + on behalf of a html response using http(s). When X-Forwarded-Proto is in
  + effect and the protocol is changed in front of the PageSpeed-enabled server,
  + a mismatch may arise when performing loopback fetches, leading up to fetching
  + failures. Explicitly authorizing the domain used to fetch resources will
  + resolve this problem, because doing so allows the oopback fetching mechanism
  + to be bypassed.
  </p>
  <p>
  