API request for adding Tests to Test Plan is not working when using Test Set Key.

Description: The POST request to add a Test set to the Test plan is not working when using the Test Set Key.

How to reproduce

1. Send an API request: http://yourserver/rest/raven/1.0/api/testplan/\ {testSetKey}

   /test 

Actual result/Attachments: Unable to add Tests to the Test Plan using Test Set Key.

Expected result: Should be able to add Tests to the Test Plan in the Test Set.