Add SVt_RV case for older Perl(< 5.11)

On newer Perl, SVt_RV equals to SVt_IV. While on older Perl,
SVt_RV does not equal to SVt_IV. So default case is executed
if first argument is reference on older Perl.
Merge pull request #6 from tokuhirom/fix-older-perl

Add SVt_RV case for older Perl(< 5.11)