Allow open_graph generates meta tag "keywords" by page.keywords, page.tags or config.keywords

[restran]

The open_graph generates HTML meta tag "keywords" only according to page.tags. Allow HTML meta tag "keywords" generated in order of page.keywords, page.tags and config.keywords could be more flexible.

// mypost.md
// custom keywords for html meta tag "keywords"
keywords: [web, site, hexo]
tags: [web, site]

If no keywords or tags specified in page, keywords in _config.js will be used.

// _config.js
keywords: blog, hexo, site

var keywords = page.keywords || (page.tags && page.tags.length ? page.tags : undefined) || config.keywords;

I also check the type of keywords to support string or array.

  if (keywords) {
    if (typeof keywords === 'string') {
      result += meta('keywords', keywords);
    } else if (keywords.length) {
      result += meta('keywords', keywords.map(function(tag) {
        return tag.name ? tag.name : tag;
      }).filter(function(keyword) {
        return !!keyword;
      }).join());
    }
  }

I also add escapeHTML to the content of meta tag in consider of avoiding XSS issue. These characters ' "
<> & / in content of meta tag should be escaped.

var escapeHTML = util.escapeHTML;

function meta(name, content, escape) {
  if (escape !== false) {
    content = escapeHTML(content);
  }

  return htmlTag('meta', {
    name: name,
    content: content
  }) + '\n';
}

function og(name, content, escape) {
  if (escape !== false) {
    content = escapeHTML(content);
  }

  return htmlTag('meta', {
    property: name,
    content: content
  }) + '\n';
}

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

[coveralls]

Coverage decreased (-0.6%) to 96.632% when pulling e3aff27 on restran:feature_open_graph into fbdee90 on hexojs:master.

[coveralls]

Coverage decreased (-0.6%) to 96.632% when pulling 800aa7e on restran:feature_open_graph into fbdee90 on hexojs:master.

[coveralls]

Coverage decreased (-0.6%) to 96.632% when pulling 800aa7e on restran:feature_open_graph into fbdee90 on hexojs:master.

[coveralls]

Coverage decreased (-0.02%) to 97.163% when pulling dc99384 on restran:feature_open_graph into fbdee90 on hexojs:master.

[coveralls]

Coverage decreased (-0.02%) to 97.163% when pulling b011003 on restran:feature_open_graph into fbdee90 on hexojs:master.

[coveralls]

Coverage decreased (-0.02%) to 97.163% when pulling e644997 on restran:feature_open_graph into fbdee90 on hexojs:master.

[NoahDragon]

Could please add the test cases?

[restran]

@NoahDragon OK, i will do it later.

[NoahDragon]

@restran Thank you. 👍

[coveralls]

Coverage increased (+0.006%) to 97.188% when pulling f41b583 on restran:feature_open_graph into fbdee90 on hexojs:master.

[coveralls]

Coverage increased (+0.006%) to 97.188% when pulling 768e567 on restran:feature_open_graph into fbdee90 on hexojs:master.

[restran]

@NoahDragon Please review.

[NoahDragon]

LGTM. Thanks for the contribution.