code: function filterKeys(collection, iteratee) {
      return _.transform(collection, function(results, val, key) {
          if (iteratee(val, key, collection)) {
              results.push(key);
          }
      }, []);
  }
docs: Iterates over keys of a collection, returning an array of all keys predicate returns truthy for.
  The predicate is invoked with three arguments: (value, index|key, collection).
  
  @static
  @memberOf _
  @category Collection
  @param {object} collection The object to iterate over.
  @param {function} iteratee The function invoked per iteration.
  @return {array} Resulting keys