better check if 'IS NOT' or '!=' should be used

lib/utils.js

@@ -299,7 +299,7 @@ var Utils = module.exports = {
     case 'eq':
       return '='
     case 'ne':
-      return val ? '!=' : 'IS NOT'
+      return val === null ? 'IS NOT' : '!='
     case 'between':
     case '..':
       return 'BETWEEN'
@@ -451,7 +451,7 @@ var Utils = module.exports = {
 
       for (var key in hash) {
         if (key instanceof Utils.literal) {
-          _hash[key] = hash[key]          
+          _hash[key] = hash[key]
         } else if (key.indexOf('.') === -1) {
           _hash[tableName + '.' + key] = hash[key]
         } else {

test/mysql/query-generator.test.js

@@ -273,6 +273,16 @@ if (Support.dialectIsMySQL()) {
           arguments: ['myTable', {where: { field: new Buffer("Sequelize")}}],
           expectation: "SELECT * FROM `myTable` WHERE `myTable`.`field`=X'53657175656c697a65';",
           context: QueryGenerator
+        }, {
+          title: 'use != if ne !== null',
+          arguments: ['myTable', {where: {field: {ne: 0}}}],
+          expectation: "SELECT * FROM `myTable` WHERE `myTable`.`field` != 0;",
+          context: QueryGenerator
+        }, {
+          title: 'use IS NOT if ne === null',
+          arguments: ['myTable', {where: {field: {ne: null}}}],
+          expectation: "SELECT * FROM `myTable` WHERE `myTable`.`field` IS NOT NULL;",
+          context: QueryGenerator
         }
       ],