Given an array and a keeper element, create a function "keep" returns an array containing the items that match the given keeper element.

keep

Notes:
* If no elements match, "keep" should return an empty array.

function keep(array, keeper) {
   if(Array.isArray(array) === false){
    return null;
  }
     return array.filter(function(value) {
        return value == keeper;
    });
}