Test JSON as Perl object, not string.

If $hashref has multiple key-value pairs, json->encode($hashref)
returns unpredictable output from Perl 5.18, because of hash
randomization. So I think that it is not good test as string.

This fixes #1 issue
Merge pull request #2 from syohex/fix-for-hash-randomization

Test JSON as Perl object, not string.