use findOrCreate in hasOne#create

So hasOne#create could take advantage from optimized findOrCreate,
which can avoid multiple creation among concurrent requests.

Signed-off-by: Clark Wang <clark.wangs@gmail.com>

lib/relation-definition.js

@@ -1539,22 +1539,17 @@ HasOne.prototype.create = function (targetModelData, cb) {
   this.definition.applyScope(modelInstance, query);
   this.definition.applyProperties(modelInstance, targetModelData);
 
-  modelTo.findOne(query, function(err, result) {
-    if(err) {
-      cb(err);
-    } else if(result) {
-      cb(new Error('HasOne relation cannot create more than one instance of '
-        + modelTo.modelName));
+  modelTo.findOrCreate(query, targetModelData, function (err, targetModel, created) {
+    if (err) {
+      return cb && cb(err);
+    }
+    if (created) {
+      // Refresh the cache
+      self.resetCache(targetModel);
+      cb && cb(err, targetModel);
     } else {
-      modelTo.create(targetModelData, function (err, targetModel) {
-        if (!err) {
-          // Refresh the cache
-          self.resetCache(targetModel);
-          cb && cb(err, targetModel);
-        } else {
-          cb && cb(err);
-        }
-      });
+      cb && cb(new Error('HasOne relation cannot create more than one instance of '
+        + modelTo.modelName));
     }
   });
 };