There has been done a change in behavior of servlet methods between 7.2 and 7.3:
For more information see related issues:
UNDERTOW-1159, UNDERTOW-1499. This behavior should be made configurable via a subsystem option (not-implemented yet) - in 7.2.1 it is configurable via a system-property, see JBEAP-16448.
Please document this change of behavior in appropriate section of our migration guide for EAP7.3.0.GA.