support specifying the field of a foreign key for mssql

The MSSQL dialect implements its own sql generator, and was accidentally
adding an extra raw attribute for an aliased field

lib/dialects/mssql/query-generator.js

@@ -138,9 +138,10 @@ module.exports = (function() {
       var modelAttributeMap = {};
       if (modelAttributes) {
         Utils._.each(modelAttributes, function(attribute, key) {
-          modelAttributeMap[key] = attribute;
           if (attribute.field) {
             modelAttributeMap[attribute.field] = attribute;
+          } else {
+            modelAttributeMap[key] = attribute;
           }
         });
       }