Fix one more "Reviewers" wire format issue

Summary: Fixes T11010. This also needs to be inflated until we fix the whole client/server responsibility issue here.

Test Plan:

• Created a revision while observing error log, no error.
• Disabled "allow self accept", tried to make myself a reviewer, got rejected with an error message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11010

Differential Revision: https://secure.phabricator.com/D15966