Changing [].flatten to array wrapper method

[rahul-tapali]

Changing [].flatten to array wrapper method

   [[1,2,3]].flatten <=> Array([1,2,3])   =>  [1,2,3]
   [1].flatten    <=>   Array(1)    => [1]

And this will avoid the nil value

   [nil].flatten   => [nil]
   Array(nil)    =>  []