Duplicate lang, language, charset meta tags when `target:static`

[abernh]

When using with target:static multiple meta tags are duplicated in my generated html files.
E.g. lang, charset, language, viewport, ... all meta tags, that modules.js are handled during generate() as "without content"

Investigating the issue I found, that this is a "problem" with vue-meta.
when vue-meta is refreshing the metas it removes duplicates only based on the given hid
If there is no hid set, then it is possible, that a single meta value is added multiple times.

Proposed fix

Adding hid based on meta.id when generating the metas so that vue-metas filtering works as expected.

//modules.js:241
  const generate = (metas, optionsGenerate, id = '', index = false) => {
      ...

      if (meta.value && meta.multi && Array.isArray(meta.value)) {
        ...
      } else if (typeof meta.value === 'object' && !Array.isArray(meta.value)) {
        ...
      } else if (meta.content) {
        ...
      } else if (meta.id) {
        if (meta.ids) {
          meta.ids.map(id => {
            ...

            outputMeta.push({
              hid: id, // -------------------> add hid
              [id]: meta.value
            })
          })
        } else {
          ...     

          outputMeta.push({
            hid: meta.id, //  -------------------> add hid
            [meta.id]: meta.value
          })
        }
      }
    })
  }

I'll prepare a PR

[TiagoDanin]

#109