Servlet 3.1 requires an ISE if getRequest() or getResponse() are called after complete() or dispatch()

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1491940 13f79535-47bb-0310-9956-ffa450edef68
Add a comment as the default is not set where one might expect.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1491941 13f79535-47bb-0310-9956-ffa450edef68
Fix typo spotted by kkolinko

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1491942 13f79535-47bb-0310-9956-ffa450edef68