add default_namespace attribute

issue #45

yaserde/tests/se_namespace.rs

@@ -161,6 +161,27 @@ fn ser_struct_default_namespace() {
   convert_and_validate!(model, content);
 }
 
+#[test]
+fn ser_struct_default_namespace_via_attribute() {
+  #[derive(YaSerialize, PartialEq, Debug)]
+  #[yaserde(
+    root = "tt",
+    default_namespace = "ttml",
+    namespace = "ttml: http://www.w3.org/ns/ttml",
+    namespace = "ttm: http://www.w3.org/ns/ttml#metadata"
+  )]
+  pub struct XmlStruct {
+    item: String,
+  }
+
+  let model = XmlStruct {
+    item: "something".to_string(),
+  };
+
+  let content = "<?xml version=\"1.0\" encoding=\"utf-8\"?><tt xmlns=\"http://www.w3.org/ns/ttml\" xmlns:ttm=\"http://www.w3.org/ns/ttml#metadata\"><item>something</item></tt>";
+  convert_and_validate!(model, content);
+}
+
 #[test]
 fn de_struct_namespace_nested() {
   #[derive(YaSerialize, Default, PartialEq, Debug)]