Fix http://issues.apache.org/bugzilla/show_bug.cgi?id=56581
If an error on a JSP page occurs when response has already been committed,
do not clear the buffer of JspWriter, but flush it.
It is backport of r1600899, r1602046, r1602583 from tomcat/trunk.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1602584 13f79535-47bb-0310-9956-ffa450edef68