Changes to POST /invitations

added the non-success response with the status 402

the response property invited_by became nullable for the status 201

the request property invited_by became nullable (media type: application/json)

the request property invited_by became nullable (media type: application/x-www-form-urlencoded)

the request property invited_by became nullable (media type: multipart/form-data)

added the new optional header request parameter X-Organization-Id

endpoint added