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
* whatwg/html#2319
* html5lib/html5lib-tests#88

All html5lib tree-construction tests are now passing:

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

Author: craigbarnes

lib/parser.c

@@ -1668,7 +1668,7 @@ static bool is_special_node(const GumboNode* node) {
       TAG(ADDRESS), TAG(APPLET), TAG(AREA), TAG(ARTICLE),
       TAG(ASIDE), TAG(BASE), TAG(BASEFONT), TAG(BGSOUND), TAG(BLOCKQUOTE),
       TAG(BODY), TAG(BR), TAG(BUTTON), TAG(CAPTION), TAG(CENTER), TAG(COL),
-      TAG(COLGROUP), TAG(MENUITEM), TAG(DD), TAG(DETAILS), TAG(DIR),
+      TAG(COLGROUP), TAG(DD), TAG(DETAILS), TAG(DIR),
       TAG(DIV), TAG(DL), TAG(DT), TAG(EMBED), TAG(FIELDSET),
       TAG(FIGCAPTION), TAG(FIGURE), TAG(FOOTER), TAG(FORM), TAG(FRAME),
       TAG(FRAMESET), TAG(H1), TAG(H2), TAG(H3), TAG(H4), TAG(H5), TAG(H6),
@@ -2379,7 +2379,7 @@ static bool handle_in_head(GumboParser* parser, GumboToken* token) {
     return handle_in_body(parser, token);
   } else if (
     tag_in(token, kStartTag, (TagSet) {
-      TAG(BASE), TAG(BASEFONT), TAG(BGSOUND), TAG(MENUITEM), TAG(LINK)
+      TAG(BASE), TAG(BASEFONT), TAG(BGSOUND), TAG(LINK)
     })
   ) {
     insert_element_from_token(parser, token);
@@ -2678,7 +2678,7 @@ static bool handle_in_body(GumboParser* parser, GumboToken* token) {
     return false;
   } else if (
     tag_in(token, kStartTag, (TagSet) {
-      TAG(BASE), TAG(BASEFONT), TAG(BGSOUND), TAG(MENUITEM), TAG(LINK),
+      TAG(BASE), TAG(BASEFONT), TAG(BGSOUND), TAG(LINK),
       TAG(META), TAG(NOFRAMES), TAG(SCRIPT), TAG(STYLE), TAG(TEMPLATE),
       TAG(TITLE)
     })

test/tree-construction.lua

@@ -152,8 +152,6 @@ do
             if test["script-on"] then
                 -- The "script-on" tests are not relevant to a non-scripted
                 -- parser, so they are just ignored instead of "skipped".
-            elseif input:find("<menuitem>") then
-                skipped = skipped + 1
             else
                 local expected = assert(test.document)
                 assert(#expected > 0)