LibWeb/XML: Set attributes using Element::set_attribute_ns

Otherwise we'd end up ignoring attr prefixes and treating them as part
of the attribute's local name.
Fixes #3137.

Libraries/LibWeb/XML/XMLDocumentBuilder.cpp

@@ -147,7 +147,11 @@ void XMLDocumentBuilder::element_start(const XML::Name& name, HashMap<XML::Name,
                 m_has_error = true;
             }
         }
-        MUST(node->set_attribute(MUST(FlyString::from_deprecated_fly_string(attribute.key)), MUST(String::from_byte_string(attribute.value))));
+        auto result = node->set_attribute_ns(m_namespace, MUST(FlyString::from_deprecated_fly_string(attribute.key)), MUST(String::from_byte_string(attribute.value)));
+        if (result.is_error()) {
+            m_has_error = true;
+            return;
+        }
     }
 
     m_current_node = node.ptr();