BigInteger: make it so you can do $min->random($max)

...and $min->randomPrime($max) as well
I think Scrutinzer is wrong. Per the default initializer list $arg2 can be false. If it is $max = $arg1. If it isn't false then $max = $arg2. So either way, $max isn't going to be false.

I mean I'm not doing any type hinting to guarantee that the non-false $max value could be a string, for example. I could do type checks before I do anything else, but then, I don't do them anywhere else in Math_BigInteger and it hasn't really posed a problem yet.

Looking at the code this can be removed:

```php
        if ($arg1 === false) {
            return false;
        }
```

$arg1 could still be false if set as such, manually, but really, this is a holdover from when it'd be false, by default, if the parameter wasn't specified. Now it's required since it's not being set to another value by default.