Stop "Header" fields (labels for form sections) from trying to generate Conduit edits

Summary: Fixes T12236. Headers are currently trying to generate an edit transaction for `maniphest.edit` and similar, but should not, since you can't edit them.

Test Plan:
  - Configured Maniphest with a custom header field.
  - Before change: `maniphest.edit` API console page fataled.
  - After change: all good, no weird "header" transaction.
  - Header still shows up on "Edit Task" form in web UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12236

Differential Revision: https://secure.phabricator.com/D17332

src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldHeader.php

@@ -34,4 +34,8 @@ final class PhabricatorStandardCustomFieldHeader
     return false;
   }
 
+  public function shouldAppearInConduitTransactions() {
+    return false;
+  }
+
 }