Updates to wildfly-undertow_1_2.xsd missing

The following commit was applied to WildFly 8 and is missing from WildFly 9: -

https://github.com/wildfly/wildfly/commit/676a8c1d0ec15bba3ebd54f44517548defdfeab8#diff-9e93e2ec05c225f1862ed049feb9abb7

Probably happened during the re-factoring.

Should also double check this attribute has not been dropped from the next version of the schema (unless of course that is intentional).