refactor(theme/view): simplify assignIn

[SukkaW]

What does it do?

The continuation of #3969 (#3753).

underscore.js has a createAssigner() which is a internal function for _.extend, _.defaults & _.assign:

_.extend = createAssigner(_.allKeys);
_.assign = createAssigner(_.keys);
_.defaults = createAssigner(_.allKeys, true);

When bring up #3969, I just simply merge and rewrite createAssigner(_.allKeys):

// Below is a part of _.allKeys which is unnecessary for assignIn itself
const keys = [];
for (const key in source) keys.push(key);

// Iterate Object from keys
const l = keys.length;	
for (let i = 0; i < l; i++) {
  const key = keys[i];
  obj[key] = source[key];
}

This PR simplify the assignIn() by directly iterating Object.

How to test

git clone -b simplify-assignIn https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

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

[coveralls]

Coverage decreased (-0.004%) to 97.114% when pulling 809cb78 on SukkaW:simplify-assignIn into fa59eb2 on hexojs:master.