Update ClassLoaderLeakPreventor.java

Update for jetty mbean problem
Merge branch 'master' of https://github.com/rapla/classloader-leak-prevention into jetty
Refactoring and bugfixing special treatment for Jetty JMX
Improved workaround for https://bugs.eclipse.org/bugs/show_bug.cgi?id=423255 that will handle cases where Jetty objects are null
Merge branch 'jetty'
Merge branch 'jetty'
[maven-release-plugin] prepare release classloader-leak-prevention-1.9.0