"javax.activation" needs to be listed in the system classes

I can't seem to reopen JETTY-420, so I'm filing this separately.

Jetty includes "javax.xml" in the list of packages that the WebAppClassLoader always consults the parent first (I'll call such packages as "pass-through" packages.) In JRE6, rt.jar now contains javax.xml.soap.AttachmentPart (a part of SAAJ API), which refers to javax.activation.DataHandler.

However, "javax.activation" is not listed as a pass-through package. As a result, if the web application directly references DataHandler and also uses SAAJ, it'll result in a classloader constraint violation. This is the root cause of JETTY-420.

Unfortunately, applications that need to run on Java5 needs to bundle activation.jar and SAAJ, thus there's nothing apps can do to avoid this bug.

In general, everything referenced by pass-through packages need to be also listed as a pass-through packages to avoid such a constraint violation.

In this case, the fix is to add "javax.activation" to the list of system classes. This definition is given in WebbAppContext._systemClasses.