function createAggregator(setter, initializer) {
      return function(collection, iteratee, thisArg) {
        iteratee = getCallback(iteratee, thisArg, 3);

        var result = initializer ? initializer() : {};
        if (isArray(collection)) {
          var index = -1,
              length = collection.length;

          while (++index < length) {
            var value = collection[index];
            setter(result, value, iteratee(value, index, collection), collection);
          }
        } else {
          baseEach(collection, function(value, key, collection) {
            setter(result, value, iteratee(value, key, collection), collection);
          });
        }
        return result;
      };
    }
Creates a function that aggregates a collection, creating an accumulator
object composed from the results of running each element in the collection
through `iteratee`. The given setter function sets the keys and values of
the accumulator object. If `initializer` is provided it is used to initialize
the accumulator object.

@private
@param {Function} setter The function to set keys and values of the accumulator object.
@param {Function} [initializer] The function to initialize the accumulator object.
@returns {Function} Returns the new aggregator function.