Auto merge of #16472 - cu1t:#14095-fix-xml-doc-namespaces, r=nox

Fix namespaces of elements created in XML documents

Correctly implement following step of [Dom Document Spec](https://dom.spec.whatwg.org/#dom-document-createelement):
> Let namespace be the HTML namespace, if the context object is an HTML document or context object’s content type is "application/xhtml+xml", and null otherwise.

Note, this will make following test in `tests/wpt/web-platform-tests/dom/nodes/Document-constructor.html` to fail, so related .ini file added to mark it as such:
```
test(function() {
  var doc = new Document();
  var a = doc.createElement("a");
  // In UTF-8: 0xC3 0xA4
  a.href = "http://example.org/?\u00E4";
  assert_equals(a.href, "http://example.org/?%C3%A4");
}, "new Document(): URL parsing")
```
I'm not very familiar with specs, but from quick look at it, I'm doubtfull that it is valid in the first place. This is an "application/xml" document, so I don't see why it should encode a.href. Firefox also fails that.

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components/script/dom/document.rs

@@ -2772,7 +2772,14 @@ impl DocumentMethods for Document {
         if self.is_html_document {
             local_name.make_ascii_lowercase();
         }
-        let name = QualName::new(ns!(html), LocalName::from(local_name));
+
+        let ns = if self.is_html_document || self.content_type == "application/xhtml+xml" {
+            ns!(html)
+        } else {
+            ns!()
+        };
+
+        let name = QualName::new(ns, LocalName::from(local_name));
         Ok(Element::create(name, None, self, ElementCreator::ScriptCreated))
     }