function createAggregator(setter, initializer) {
  return function(collection, iteratee) {
    var func = isArray(collection) ? arrayAggregator : baseAggregator,
        accumulator = initializer ? initializer() : {};

    return func(collection, setter, baseIteratee(iteratee, 2), accumulator);
  };
}
Creates a function like `_.groupBy`.

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