fix erroneous debug assertion in nested namespace case

The assertion was checking to ensure that the emitted tag
had no prefix, which breaks the case where a prefix is
required, but only in debug builds!

instant-xml-macros/src/ser.rs

@@ -228,7 +228,6 @@ fn serialize_struct(
             ) -> Result<(), instant_xml::Error> {
                 // Start tag
                 let prefix = serializer.write_start(#tag, #default_namespace)?;
-                debug_assert_eq!(prefix, None);
 
                 // Set up element context, this will also emit namespace declarations
                 #context

instant-xml/tests/ser-child-ns.rs

@@ -47,3 +47,34 @@ fn struct_child_namespaces() {
         "<StructChildNamespaces xmlns=\"URI\" xmlns:bar=\"BAZ\" xmlns:foo=\"BAR\"><NestedDifferentNamespace xmlns=\"\" xmlns:internal=\"INTERNAL\"><internal:flag_internal_prefix>false</internal:flag_internal_prefix></NestedDifferentNamespace><Nested xmlns:internal=\"INTERNAL\"><bar:flag_parent_prefix>true</bar:flag_parent_prefix><internal:flag_internal_prefix>false</internal:flag_internal_prefix></Nested></StructChildNamespaces>"
     );
 }
+
+#[derive(Debug, ToXml)]
+#[xml(rename = "DIDL-Lite", ns("DIDL", upnp = "UPNP"))]
+pub struct DidlLite {
+    pub item: UpnpItem,
+}
+
+#[derive(Debug, ToXml)]
+#[xml(rename = "item", ns("DIDL"))]
+pub struct UpnpItem {
+    pub album_art: Option<AlbumArtUri>,
+}
+
+#[derive(Debug, ToXml)]
+#[xml(rename = "albumArtURI", ns("UPNP", upnp = "UPNP"))]
+pub struct AlbumArtUri {
+    #[xml(direct)]
+    pub uri: String,
+}
+
+#[test]
+fn test_didl() {
+    let didl = DidlLite {
+        item: UpnpItem {
+            album_art: Some(AlbumArtUri {
+                uri: "http://art".to_string(),
+            }),
+        },
+    };
+    assert_eq!(to_string(&didl).unwrap(), "<DIDL-Lite xmlns=\"DIDL\" xmlns:upnp=\"UPNP\"><item><upnp:albumArtURI>http://art</upnp:albumArtURI></item></DIDL-Lite>");
+}