HttpUnit doesn't properly shut down the connection, causing the queue on Jetty...

HttpUnit doesn't properly shut down the connection, causing the queue on Jetty to run out. Switching to HtmlUnit, as the project seems more actively maintained.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@11481 71c3de6d-444a-0410-be80-ed276b4c234a