use defaults instead of extends for dialect support

lib/dialects/mariadb/index.js

@@ -5,7 +5,7 @@ var MariaDialect = function(sequelize) {
   this.sequelize = sequelize
 }
 
-MariaDialect.prototype = _.extend({
+MariaDialect.prototype = _.defaults({
 
 }, MySQL.prototype)
 

lib/dialects/mysql/index.js

@@ -5,8 +5,8 @@ var MysqlDialect = function(sequelize) {
   this.sequelize = sequelize
 }
 
-MysqlDialect.prototype.supports = _.extend(Abstract.prototype.supports, {
+MysqlDialect.prototype.supports = _.defaults({
 	'VALUES ()': true
-})
+}, Abstract.prototype.supports)
 
 module.exports = MysqlDialect

lib/dialects/postgres/index.js

@@ -5,9 +5,9 @@ var PostgresDialect = function(sequelize) {
   this.sequelize = sequelize
 }
 
-PostgresDialect.prototype.supports = _.extend(Abstract.prototype.supports, {
+PostgresDialect.prototype.supports = _.defaults({
 	'RETURNING': true,
 	'DEFAULT VALUES': true
-})
+}, Abstract.prototype.supports)
 
 module.exports = PostgresDialect

lib/dialects/sqlite/index.js

@@ -5,9 +5,9 @@ var SqliteDialect = function(sequelize) {
   this.sequelize = sequelize
 }
 
-SqliteDialect.prototype.supports = _.extend(Abstract.prototype.supports, {
+SqliteDialect.prototype.supports = _.defaults({
 	'DEFAULT': false,
 	'DEFAULT VALUES': true
-})
+}, Abstract.prototype.supports)
 
 module.exports = SqliteDialect