Fix bug in division of huge numbers

[aseaday]

The // will be more precise in this condition.

[tooooolong]

@Alir3z4 When encode/decode a big int, will get a wrong result. Here is a example:

>>> encoder.enbase(a)
'ueqf0wgbq4k0k'
>>> encoder.debase('ueqf0wgbq4k0k')
9223372036854775304
>>> a
9223372036854775808

And with this fix

>>> encode_url(9223372036854775304)
'pamj78cznbvrt'
>>> decode_url('pamj78cznbvrt')
9223372036854775304

[Alir3z4]

@aseaday Thanks for the patch and fixing this bug.
@tooooolong Thank you for providing show case.

I'll merge this right away ;)