Revert "Revert "expand path before adding it to the $CLASSPATH variable""

This reverts commit 59c9f74f81701eb77d66400994873099ccf30c72.
Revert "Revert "get the $CLASSPATH variable right and allow proper URL as well""

This reverts commit b555717929916000089e0769711d4251b9fe0e82.
not using lib/jruby.jar to test $CLASSPATH

adding lib/jruby.jar on the fly to Thread.currentThread.contextClassLoader (CCL)
where is parent has already lib/jruby.jar is not an good idea especially
if the CCL is the JRubyClassLoader looking first into itself before going to
its parent CL to load classes.

further I added test to ensure that the added $CLASSPATH entry indeed
is part of the classloader.

fixes #2276