Fix/handle conversion with leading zeros

This fixes conversion of numerical partial-data with leading zeroes to iban. It simply checks the targeted format-rule (e.g. to decimal 'd') and typecasts the partial-value to integer, so there won't be any mis-interpretation as octal (as already mentioned in https://github.com/alphasights/iban-tools/pull/7 by @descala).
By checking for the target format, there shouldn't be any incompatibilities with iban-partials with characters in it, though they should be already formatted as string (e.g. "%s").

If there is any other conversion-rule given with alphanumeric partials, i would also provide a contra-test, that they won't break.