Cannot specify additional classpath entries in manifest when using addClasspath

When using addClasspath, e.g. in the maven-jar-plugin, it is not possible to add additional classpath entries using manifestEntries, as this generates an illegal manifest file (contains two Class-Path entries). Please see http://jira.codehaus.org/browse/MJAR-41

I have been looking through the code, and it seems this might need to be resolved in MavenArchiver? I've attached a simple fix that solves the problem for me, but a more thorough solution might be needed of course.