correctly handle overloading

comparing references with '==' to check if they're the same fails if
they're blessed and overload numification or numeric comparison;
Scalar::Util::refaddr works

stringifying a blessed reference to get the underlying reference type
fails if it overloads stringification; Scalar::Util::reftype works
merge dakkar's https://github.com/DrHyde/perl-modules-Data-Compare/pull/2
Merge branch 'dakkar-master'