Handling of corner cases for 'fn.concat'

`fn.concat(6, [1, 2, 3])` or `fn.concat()` worked differently from how I would expect them to work. In both cases an exception would be thrown. At the same time it is perfectly fine to call standard methods like `arr1.concat(6)` and `arr1.concat()`, in the first case it will just append `6` and in the second case will leave the array unchanged. 

I added a few test cases covering those corner cases and modified the implementation of `fn.concat` a bit to handle them.

Basically I just looked at the following examples for `Array.prototype.concat` and `String.prototype.concat` and made sure that the behavior is similar for `fn.concat`:

```javascript
var arr1 = [1, 2, 3];
var arr2 = [4, 5, 6];

console.log(arr1.concat()); //[1, 2, 3]
console.log(arr1.concat(null)); //[1, 2, 3, null]
console.log(arr1.concat((void 0))); //[1, 2, 3, undefined]
console.log(arr1.concat("4")); //[1, 2, 3, "4"]
console.log(arr1.concat(arr2)); //[1, 2, 3, 4, 5, 6]

var str1 = "123";
var str2 = "456";

console.log(str1.concat(str2)); //"123456"
console.log("".concat(str2)); //"456"
console.log(str1.concat("")); //"123" 
```

By the way, I assumed that `fn.concat` should also work with strings, not only arrays. 

Please, let me know if you have any notes or suggestions.