added test for optional callbacks. cc #135

tests/test-optional-callback.js

@@ -0,0 +1,10 @@
+var assert = require('assert');
+var mongojs = require('../index');
+var db = mongojs('test', ['a','b']);
+
+db.a.ensureIndex({hello:'world'})
+setTimeout(function() {
+	db.a.count(function() {
+		db.close();
+	})
+}, 100);