Mark external -> external redirects in Phame to canonicalize URIs as "external"

Summary: Ref T9897. If you visit `/post/123/spoderman/` it will try to redirect you to `/post/123/spiderman/`, but currently only internal views work because these redirects aren't marked as safe/external.

Test Plan: Visited a misspelled/out-of-date URI on an external blog view, got a good redirect.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9897

Differential Revision: https://secure.phabricator.com/D14741

src/applications/phame/controller/PhameLiveController.php

@@ -129,7 +129,14 @@ abstract class PhameLiveController extends PhameController {
           $uri = $post->getViewURI();
         }
 
-        return id(new AphrontRedirectResponse())->setURI($uri);
+        $response = id(new AphrontRedirectResponse())
+          ->setURI($uri);
+
+        if ($is_external) {
+          $response->setIsExternal(true);
+        }
+
+        return $response;
       }
     }