fix(postPermalinkFilter): permalink_defaults should not overwrite values

[dailyrandomphoto]

What does it do?

permalink_defaults option does not work correctly from 4.1.0.
According https://hexo.io/docs/permalinks, set _config.yml as

permalink: :lang/:title/
permalink_defaults:
  lang: en

$ hexo new "Hello World" --lang tw

and the URL will be:

# expected is
http://localhost:4000/tw/hello-world/

# but actual is 
http://localhost:4000/en/hello-world/

Compare v4.0.0 and v4.1.0,
I found the related change is e0f2478#diff-dbb03cb234189f8e709e7c5f9921aa67L45 (#3790)

- return permalink.stringify(defaults(meta, config.permalink_defaults));
+ return permalink.stringify(Object.assign(meta, config.permalink_defaults));

If change to

Object.assign([], config.permalink_defaults, meta)

it causes "Maximum call stack size exceeded" error, so use for loop to copy values.

How to test

git clone -b fix-permalinkDefaults https://github.com/dailyrandomphoto/hexo.git
cd hexo
npm install
npm test

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage increased (+0.004%) to 97.135% when pulling 2e8ec74 on dailyrandomphoto:fix-permalinkDefaults into c19096b on hexojs:master.

[coveralls]

Coverage increased (+0.004%) to 97.135% when pulling 2e8ec74 on dailyrandomphoto:fix-permalinkDefaults into c19096b on hexojs:master.

[curbengh]

👍

[curbengh]

Verified on a test blog.

[SukkaW]

Try deepMerge() to see if Maximum call stack size exceeded still occurs.

---

Update

According to the documents of lodash.defaults:

Source objects are applied from left to right. Once a property is set, additional values of the same property are ignored.

Yeah, it is my mistake to replace lodash.defaults with Object.assign.