Fixing Issue Where Some Hashes Cannot Be Decoded

Fix for issue #7 and new test case
I'll make a new subtest, but the behavior is the same even with default constructor (I should've removed the custom values from my test script to avoid confusion):
```
perl -MHashids -e '$h = Hashids->new(); foreach(1..100) { print "$_\n" if($h->decode($h->encode($_)) != $_) }'
43
87
```