valuetest: add test for Uint64 comparisons
GenericValue: fix comparison of (Ui|I)nt64 numbers


Some 64-bit integers cannot be represented losslessly as a double.
Due to a typo in the operator==, the comparison has been performed
after a double conversion in too many cases.
Merge pull request #129 from pah/bug/compare-uint64-double

Failing comparisons between certain (Ui|I)nt64 numbers