Query#findOne callback is optional

allows for more flexibility when passing queries around

closes Automattic#581

lib/query.js

@@ -843,13 +843,16 @@ Query.prototype.each = function (callback) {
 Query.prototype.findOne = function (callback) {
   this.op = 'findOne';
 
+  if (!callback) return this;
+
   var model = this.model;
   var promise = new Promise(callback);
 
   try {
     this.cast(model);
   } catch (err) {
-    return promise.error(err);
+    promise.error(err);
+    return this;
   }
 
   var self = this

test/query.test.js

@@ -730,6 +730,17 @@ module.exports = {
     setTimeout(db.close.bind(db), 300);
   },
 
+  '#findOne should set the op when callback is passed': function () {
+    var db = start();
+    var query = new Query();
+    var Product = db.model('Product');
+    var q = new Query().bind(Product, 'distinct');
+    q.op.should.equal('distinct');
+    q.findOne();
+    q.op.should.equal('findOne');
+    db.close();
+  },
+
   'querying/updating with model instance containing embedded docs should work (#454)': function () {
     var db = start();
     var Product = db.model('Product');