Fix call to undefined method PhutilJSON::encodeAsObject()

Summary:
Replace call to undefined method `PhutilJSON::encodeAsObject()` with existing `PhutilJSON::encodeFormatted()`

```
EXCEPTION: (Error) Call to undefined method PhutilJSON::encodeAsObject() at [<phabricator>/src/applications/transactions/xaction/PhabricatorEditEngineDefaultTransaction.php:68]
```

Closes T15603

Test Plan: Grep and read the code in https://we.phorge.it/source/arcanist/browse/master/src/parser/PhutilJSON.php

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15603

Differential Revision: https://we.phorge.it/D25391

Author: aklapper

Diff for: src/applications/transactions/xaction/PhabricatorEditEngineDefaultTransaction.php

@@ -65,7 +65,7 @@ private function renderDefaultValueAsFallbackText($default_value) {
       return id(new PhutilJSON())->encodeAsList($default_value);
     }
 
-    return id(new PhutilJSON())->encodeAsObject($default_value);
+    return id(new PhutilJSON())->encodeFormatted($default_value);
   }
 
 }