Update randomElements to return random number of elements when no count is provided (#88)

Author: calebdw

docs/formatters/numbers-and-strings.md

@@ -92,13 +92,17 @@ echo $faker->randomLetter();
 
 ## `randomElements`
 
-Returns `$count` amount of random element from the given array, traversable, or enum. By default, the `$count` parameter is set to 1.
+Returns `$count` amount of random element from the given array, traversable, or enum. By default, the `$count` parameter is set to 1, when `null` a random number of elements is returned.
 
 ```php
 echo $faker->randomElements(['a', 'b', 'c', 'd', 'e']);
 
 // ['c']
 
+echo $faker->randomElements(['a', 'b', 'c', 'd', 'e'], null);
+
+// ['c', 'a', 'e']
+
 echo $faker->randomElements(new \ArrayIterator(['a', 'b', 'c', 'd', 'e']));
 
 // ['c']