Fix editing of JSON config values via web UI

Summary: We should set the value if it's valid, not if it's invalid. derpp~

Test Plan: Set `files.viewable-mime-types`.

Reviewers: asherkin, codeblock, btrahan, vrana

Reviewed By: asherkin

CC: aran

Differential Revision: https://secure.phabricator.com/D4510

src/applications/config/controller/PhabricatorConfigEditController.php

@@ -296,6 +296,7 @@ final class PhabricatorConfigEditController
           $errors[] = pht(
             'The given value must be valid JSON. This means, among '.
             'other things, that you must wrap strings in double-quotes.');
+        } else {
           $set_value = $json;
         }
         break;