Fix numeric code types, they are numeric characters, not ints. Leadin…

[oldpec]

Numeric codes are numeric characters, but not ints, leading zeroes matter.
Without this change both:

$isoCodes = new IsoCodesFactory();
$country = $countries->getByNumericCode('036');

and

$isoCodes = new IsoCodesFactory();
$country = $countries->getByNumericCode(36);

fail, because the map generated by \Sokil\IsoCodes\AbstractNotPartitionedDatabase::getIndex() has string array keys like '036' which doesn't match for numeric codes with leading zeroes that has been cast to an int

[sokil]

fixed in 3.1.0