Duplicate character prevention bug

[gdebrauwer]

The encoding process prevented duplicate characters in the output string. This meant it removed every used character from the character list. This causes certain numbers to have the same encoding result. This PR removes that logic, so each number is encoded correctly.

If you already use this package, this means every generated unique-code will change. By changing the encoding logic, the encoded result of each number will change, which can create collisions with older encoded numbers. You must change the length of all new codes, as this ensures every newly generated unique-code will never collide with older unique-codes.

[gdebrauwer]

We should probably tag this as a 2.0.0, otherwise, projects that already use the package might automatically upgrade.

[markvesterskov]

Just chiming in to be notified when 2.0 is tagged, using this for an upcoming app. Thanks for a useful package!

[lduhejian]

    protected function getMaximumUniqueCodes()
    {
        $maxCombinations = 1;

        for ($i = 0; $i < $this->length; $i++) {
            $maxCombinations = $maxCombinations * (strlen($this->characters) - $i);
        }

        return $maxCombinations;
    }

This function might also needs to be modified.

    protected function getMaximumUniqueCodes()
    {
        return pow(strlen($this->characters),$this->length);
    }