[Fixes #75802914] Allow whitespace in X-Forwarded-For

The draft specification, which is probably the best we'll ever have, says
that whitespace between values is optional:

- http://tools.ietf.org/html/draft-ietf-appsawg-http-forwarded-10#section-7.1

Although it's quite common for there to be spaces, CloudFlare and some other
reverse-proxies don't add them, and it's even more common for software that
reads the header to do a split+trim.

So relax this test by checking the total number of values and then the value
of each after stripping whitespace.
Would it help to include the full contents of the header in this message? It might make it easier to troubleshoot any issues.