Allow non-String args in JOptCommandLinePropertySource

Prior to this commit, JOptCommandLinePropertySource prevented the
possibility of non-String option arguments. This effectively prevents
the use of JOpt's #ofType support (which allows specifying custom
argument types).

Now, non-String arguments are detected and converted to strings as
necessary. JOpt's #ofType now works as expected. A test has been added
to cover this case.