[MSSQL] Fix connection manager domain usage

API for the Tedious driver requires that an entry for `domain`, if present,
be at the root level of the config option passed to the constructor. This
was added in commit 91fc76be.

However, that commit deleted dialectOptions.domain from the config object
passed into the Sequelize constructor. Since most config objects are singletons,
this mutation of the config object breaks any subsequent calls to the instance
connectionManager.$connect.

This commit simply removes the call to delete the domain entry.
Fixes #4918

lib/dialects/mssql/connection-manager.js

@@ -53,7 +53,6 @@ ConnectionManager.prototype.connect = function(config) {
       // The 'tedious' driver needs domain property to be in the main Connection config object
       if(config.dialectOptions.domain) {
         connectionConfig.domain = config.dialectOptions.domain;
-        delete config.dialectOptions.domain;
       }
 
       Object.keys(config.dialectOptions).forEach(function(key) {

test/unit/dialects/mssql/connection-manager.js

+'use strict';
+
+var chai = require('chai')
+  , expect = chai.expect
+  , Sequelize = require(__dirname + '/../../../../index');
+
+
+var tedious = require('tedious')
+  , sinon = require('sinon')
+  , connectionStub = sinon.stub(tedious, 'Connection');
+
+connectionStub.returns({on: function () {}});
+
+describe('[MSSQL] Connection Manager', function () {
+  var instance
+    , config;
+  beforeEach(function () {
+    config = {
+      dialect: 'mssql',
+      database: 'none',
+      username: 'none',
+      password: 'none',
+      host: 'localhost',
+      port: 2433,
+      pool: {},
+      dialectOptions: {
+        domain: 'TEST.COM'
+      }
+    };
+    instance = new Sequelize(config.database
+                             , config.username
+                             , config.password
+                             , config);    
+  });
+  
+  it('connectionManager.$connect() Does not delete `domain` from config.dialectOptions',
+     function () {
+       expect(config.dialectOptions.domain).to.equal('TEST.COM');
+       instance.dialect.connectionManager.$connect(config);
+       expect(config.dialectOptions.domain).to.equal('TEST.COM');
+     });
+});