publisher: Fix tag collector for nested table elements

[bep]

Fixes #7318

[jeromew]

I tried to understand why travis fails on this PR.

it says

quicktest.go:306: 
        error:
          unexpected length
        len(got):
          int(9)
        got:
          []string{"!doctype", "div", "head", "hr", "html", "link", "meta", "span", "title"}
        want length:
          int(8)
        stack:
          /home/travis/src/hugo/hugolib/site_test.go:1126
            b.Assert(els.Tags, qt.HasLen, 8)

Looking at the test, I can only guess that it is because <!doctype> was implicitly added, was impersonated as a div by the PR and as a consequence was added to the list of elements (?).
I hope this helps.

[github-actions]

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