Fix InvokeParameterisedMethod in handling varargs

The old `testMethodParamsAreVarargs` assumes that varargs will be the only argument in a function, while it might not be necessarily so. Other arguments may present, provided that varargs is the last argument, as specified in http://docs.oracle.com/javase/1.5.0/docs/guide/language/varargs.html

Excerpts from that page:
> Varargs can be used only in the final argument position.