Fixed PR #192 (multipleOf decimal with big integer value)

[linco95]

#192

The issue seemed to be that number | 0 !== number to detect integers doesn't work for big numbers. I replaced it with the Number.isInteger function, which seems to handle all test cases quite well

Also inverted the ternary operator to avoid the negation. I also suggest replacing multipleOf.toString().split('.').pop().length with a named function

const getDecimalPart = (num) => num.toString().split(".").pop();
const getNumberOfDecimals = (num) => getDecimalPart(num).length;

var factor = Number.isInteger(multipleOf) ? 1 : Math.pow(10, getNumberOfDecimals(multipleOf))

[linco95]

Closes #192