fix(write): correct export of generic element

* Properly read parent declared namespace
* Properly register local non-prefix namespace

Author: nikku

lib/write.js

@@ -66,7 +66,9 @@ Namespaces.prototype.add = function(ns, isWellknown) {
 };
 
 Namespaces.prototype.uriByPrefix = function(prefix) {
-  return this.prefixMap[prefix || 'xmlns'];
+  return this.prefixMap[prefix || 'xmlns'] || (
+    this.parent && this.parent.uriByPrefix(prefix)
+  );
 };
 
 Namespaces.prototype.mapPrefix = function(prefix, uri) {
@@ -463,7 +465,7 @@ ElementSerializer.prototype.parseNsAttribute = function(element, name, value) {
  * @param  {Object} element
  * @return {Array<Object>}
  */
-ElementSerializer.prototype.parseNsAttributes = function(element, attrs) {
+ElementSerializer.prototype.parseNsAttributes = function(element) {
   var self = this;
 
   var genericAttrs = element.$attrs;
@@ -622,6 +624,11 @@ ElementSerializer.prototype.logNamespaceUsed = function(ns, local) {
 
   ns = namespaces.byUri(uri);
 
+  // register new default prefix <xmlns> in local scope
+  if (!ns && !prefix) {
+    ns = this.logNamespace({ uri }, wellknownUri === uri, true);
+  }
+
   if (!ns) {
     newPrefix = prefix;
     idx = 1;
@@ -653,8 +660,7 @@ ElementSerializer.prototype.parseAttributes = function(properties) {
 
       if (!p.isMany) {
         value = value.id;
-      }
-      else {
+      } else {
         var values = [];
         forEach(value, function(v) {
           values.push(v.id);

test/spec/rountrip.js

@@ -175,6 +175,33 @@ describe('Roundtrip', function() {
 
   describe('generic', function() {
 
+    it('should keep local ns attribute', async function() {
+
+      // given
+      var extendedModel = createModel([ 'extensions' ]);
+
+      var reader = new Reader(extendedModel);
+      var writer = createWriter(extendedModel);
+
+      var rootHandler = reader.handler('e:Root');
+
+      var input =
+        '<e:root xmlns:e="http://extensions" xmlns:woop="https://woop">' +
+          '<Bar xmlns="http://foobar">' +
+            '<Foo woop:boop="Some" />' +
+          '</Bar>' +
+        '</e:root>';
+
+      // when
+      var {
+        rootElement
+      } = await reader.fromXML(input, rootHandler);
+
+      var output = writer.toXML(rootElement);
+
+      // then
+      expect(output).to.eql(input);
+    });
 
 
     it('should keep local <xsi:type>', async function() {