fix(open_graph): locale must in language_TERRITORY format

[curbengh]

What does it do?

Recently tested hexo.io against facebook linter. Apparently, the value for "og:locale" must be in language_TERRITORY format, so "en" is invalid. Here is a list of valid values.

If og:locale is not specified, fb defaults to "en_US".

(The following note is only applicable if you're not using en-US locale. It's optional for en-US users.)

If the language value (regardless from config or front matter) is not in language-TERRITORY format, "og:locale" will not be added.

If the language value in _config is just two characters (e.g. "en", "fr"), then you need to append territory value, e.g. "en-GB", "fr-CA". you also need to update the filename of the language file (e.g. "en.yml" -> "en-GB.yml") of your theme.

Alternatively, you could also set <%- open_graph({language: 'en-GB'}) %> in your theme layout.

How to test

git clone -b ogp-locale https://github.com/curbengh/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 (-0.1%) to 97.144% when pulling 5751b12 on curbengh:ogp-locale into 71d91af on hexojs:master.

[curbengh]

Removed i/"ignore case" regex flag since language variable is always derived from language variable, so not only the flag is unnecessary, it's actually safer to match case (for this purpose).