Remove special handling for <menuitem> tag
Remove special handling for <menuitem> tag

It has been removed from the whatwg HTML spec and the html5lib
tests now expect it to be handled as an unknown element.

See also:

* https://developer.mozilla.org/en-US/docs/Web/HTML/Element/menuitem
* https://github.com/whatwg/html/pull/2319
* https://github.com/html5lib/html5lib-tests/pull/88

All html5lib tree-construction tests are now passing:

    Ran 1689 tests in 0.15s
    Passed: 1689
    Failed: 0
    Skipped: 0

rubys/nokogumbo,rubys/nokogumbo,rubys/nokogumbo,rubys/nokogumbo