Map default xml prefix

instant-xml/src/de.rs

@@ -185,6 +185,13 @@ impl<'xml> Context<'xml> {
     }
 
     fn lookup(&self, prefix: &str) -> Option<&'xml str> {
+        // The prefix xml is by definition bound to the namespace
+        // name http://www.w3.org/XML/1998/namespace
+        // See https://www.w3.org/TR/xml-names/#ns-decl
+        if prefix == "xml" {
+            return Some("http://www.w3.org/XML/1998/namespace");
+        }
+
         self.stack
             .iter()
             .rev()

instant-xml/tests/de-ns.rs

@@ -34,6 +34,12 @@ fn default_namespaces() {
         Ok(NestedDe { flag: true })
     );
 
+    // Default namespace not-nested - with xml:lang
+    assert_eq!(
+        from_str("<NestedDe xml:lang=\"en\" xmlns=\"URI\" xmlns:bar=\"BAZ\"><bar:flag>true</bar:flag></NestedDe>"),
+        Ok(NestedDe { flag: true })
+    );
+
     // Default namespace not-nested - wrong namespace
     assert_eq!(
         from_str(