Fix preconnect config for vendors with custom cdn

[leirock]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

If we set venders.custom_cdn_url: https://lib.baomitu.com/${cdnjs_name}/${version}/${cdnjs_file}, the preconnect in head will be <link rel="preconnect" href="lib.baomitu.com" crossorigin="">

And in Safari, the console shows Successfully preconnected to https://pinlyu.com/lib.baomitu.com

See: https://pinlyu.com

What is the new behavior?

• For HTTPS CDN

If we set venders.custom_cdn_url: https://lib.baomitu.com/${cdnjs_name}/${version}/${cdnjs_file}, the preconnect in head will be <link rel="preconnect" href="https://lib.baomitu.com" crossorigin="">

And in Safari, the console will show Successfully preconnected to https://lib.baomitu.com/

• For HTTP CDN

If we set venders.custom_cdn_url: http://lib.baomitu.com/${cdnjs_name}/${version}/${cdnjs_file}, the preconnect in head will be <link rel="preconnect" href="http://lib.baomitu.com" crossorigin="">

And in Safari, the console will show Successfully preconnected to http://lib.baomitu.com/

[coveralls]

Pull Request Test Coverage Report for Build 2242555929

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.468%

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Totals
Change from base Build 2216190865:	0.0%
Covered Lines:	138
Relevant Lines:	141

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💛 - Coveralls

[stevenjoezhang]

Does Line 61

local   : parse(this.theme.js || '').hostname,

need to be updated?

See also #439

CC @ljcbaby

[leirock]

Yep, seems Line 61 also needs to be updated.

[ljcbaby]

@stevenjoezhang I see my html before, it may generated fine. https://github.com/ljcbaby/ljcbaby.github.io/blob/master/index.html#L947-L950

Forget it, I seems to be not awake now, and see the wrong place.