[JENKINS-10851] Reverting part of 1fd7fb2c : excluding xalan because its presence makes Jenkins KO on IBM JVM.

integrating newer version of HtmlUnit to fix
memory inefficiency problem.

http://jira.codehaus.org/browse/GROOVY-5003

Use stock gmaven plugin to achieve this

happen.

This happens because the version number isn't computed yet, but we try
to put that into HTTP header.

Fixing this by making <st:header/> tolerate null in value or name.

This reverts commit 0ef9424d.