Fixed incorrect GenericParameter reference in JSIL.CloneParameter (#579)

I've fixed issue and doesn't break any tests. Still it will be good to review, what should return `EmitThisForParameter` if `CurrentMethod == null`.
Currently I make it return true, as it works before my patch. You could check on which condition `CurrentMethod == null` through SimpleTestCasesForTranslatedBcl\ExpressionsTest.