API login as user improvements

The BILL partner API endpoints require sessionId as a header value that is generated with API partner login (POST /v3/partner/login). We learned that API login as user (POST /v3/partner/login-as-user) endpoint is the only BILL partner endpoint where sessionId is required as a body parameter.

For consistency across the BILL partner API endpoints, we now require sessionId as a header value for the API login as user endpoint. See API login as user in the API reference for more information.