Join tokens when stringifying DOMTokenList

Previous, it would return the original String straight from the
AttrValue, which might contain extraaneous whitespace. The spec
specifies to just join the tokens together with \x20

https://dom.spec.whatwg.org/#stringification-behavior

components/script/dom/domtokenlist.rs

@@ -157,6 +157,11 @@ impl<'a> DOMTokenListMethods for &'a DOMTokenList {
 
     // https://dom.spec.whatwg.org/#stringification-behavior
     fn Stringifier(self) -> DOMString {
-        self.element.root().r().get_string_attribute(&self.local_name)
+        let tokenlist = self.element.root().r().get_tokenlist_attribute(&self.local_name);
+        tokenlist.iter().fold(String::new(), |mut s, atom| {
+            if !s.is_empty() { s.push('\x20'); }
+            s.push_str(atom);
+            s
+        })
     }
 }

tests/wpt/metadata/dom/lists/DOMTokenList-stringifier.html.ini

@@ -1,5 +0,0 @@
-[DOMTokenList-stringifier.html]
-  type: testharness
-  [DOMTokenList stringifier]
-    expected: FAIL
-

tests/wpt/metadata/dom/nodes/Element-classlist.html.ini

@@ -3,9 +3,6 @@
   [CSS .foo selectors must not match elements without any class]
     expected: FAIL
 
-  [empty classList should return the empty string since the ordered set parser skip the whitespaces]
-    expected: FAIL
-
   [computed style must update when setting .className]
     expected: FAIL