DataTransferObjectError::invalidType : get actual type before mutating $value for the error message

[mfn]

I believe this is an error and wasn't caught so far because no tests for the actual message did exist.

In \Spatie\DataTransferObject\DataTransferObjectError::invalidType this code was executed:

        $currentType = gettype($value);

but it was done so after $value was possible overwritten like this:

        if ($value === null) {
            $value = 'null';
        }

        if (is_object($value)) {
            $value = get_class($value);
        }

        if (is_array($value)) {
            $value = 'array';
        }

Which basically made gettype($value); to yield string in the majority of cases.

This PR fixes this.
Please also see the 2nd commit stand-alone, showing the change form the old to the new message => 182c925

[mfn]

Before this PR, this message ended in:

instead got value null, which is string

and now

instead got value null, which is NULL

[brendt]

Released as 1.13.2: https://github.com/spatie/data-transfer-object/releases/tag/1.13.2

Thanks for all the PRs!