Fix: ensure that APIs for comments return with user.isGravatarEnabled field

lib/models/comment.js

@@ -2,7 +2,7 @@ module.exports = function(crowi) {
   var debug = require('debug')('crowi:models:comment')
     , mongoose = require('mongoose')
     , ObjectId = mongoose.Schema.Types.ObjectId
-    , USER_PUBLIC_FIELDS = '_id image googleId name username email status createdAt' // TODO: どこか別の場所へ...
+    , USER_PUBLIC_FIELDS = '_id image isGravatarEnabled googleId name username email status createdAt' // TODO: どこか別の場所へ...
     , commentSchema
   ;