Add trust for proxy prefixed back in

Was not set explicitly before, but a bug in Laravel caused prefixes to
be trusted in our middlware anyway. (Fixed in v10.17.0).
This adds it back in explicitly so the header is properly used again.

app/Http/Middleware/TrustProxies.php

@@ -24,5 +24,6 @@ class TrustProxies extends Middleware
         Request::HEADER_X_FORWARDED_HOST |
         Request::HEADER_X_FORWARDED_PORT |
         Request::HEADER_X_FORWARDED_PROTO |
+        Request::HEADER_X_FORWARDED_PREFIX |
         Request::HEADER_X_FORWARDED_AWS_ELB;
 }