docs(Collection): updated Collection.aggregate()

Changed pipeline parameter to optional

lib/collection.js

@@ -1682,7 +1682,7 @@ Collection.prototype.findAndRemove = function(query, sort, options, callback) {
 /**
  * Execute an aggregation framework pipeline against the collection, needs MongoDB >= 2.2
  * @method
- * @param {object} pipeline Array containing all the aggregation framework commands for the execution.
+ * @param {object} [pipeline=[]] Array containing all the aggregation framework commands for the execution.
  * @param {object} [options] Optional settings.
  * @param {(ReadPreference|string)} [options.readPreference] The preferred read preference (ReadPreference.PRIMARY, ReadPreference.PRIMARY_PREFERRED, ReadPreference.SECONDARY, ReadPreference.SECONDARY_PREFERRED, ReadPreference.NEAREST).
  * @param {object} [options.cursor] Return the query as cursor, on 2.6 > it returns as a real cursor on pre 2.6 it returns as an emulated cursor.