fix(open_graph): set default locales

[curbengh]

What does it do?

#3808 introduces a 🐔 🥚 issue; setting "language" (like "en") prevent og:locale from being inserted, setting "language-TERRITORY" (like "en-GB") breaks theme's multilingual/i8n support.

This PR fixes that by assuming default locales.
Based on Facebook's official list.

How to test

git clone -b og-locale-backward https://github.com/curbengh/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 97.14% when pulling 9aceb2f on curbengh:og-locale-backward into 979d1f4 on hexojs:master.