Improve how XmlStringBuilder handles enclosing XML environments

to avoid emitting unnecessary attributes like xmlns.

Also add a test for MediaElement that checks that <uri/> does not
carry a xmlns attribute.

smack-core/src/main/java/org/jivesoftware/smack/util/XmlStringBuilder.java

@@ -56,9 +56,17 @@ public class XmlStringBuilder implements Appendable, CharSequence, Element {
         halfOpenElement(e.getElementName());
     }
 
-    public XmlStringBuilder(FullyQualifiedElement ee, XmlEnvironment enclosingXmlEnvironment) {
+    public XmlStringBuilder(FullyQualifiedElement element, XmlEnvironment enclosingXmlEnvironment) {
-        this(enclosingXmlEnvironment);
+        sb = new LazyStringBuilder();
-        prelude(ee);
+        halfOpenElement(element);
+        if (enclosingXmlEnvironment != null
+                && !enclosingXmlEnvironment.effectiveNamespaceEquals(element.getNamespace())) {
+            xmlnsAttribute(element.getNamespace());
+        }
+        effectiveXmlEnvironment = XmlEnvironment.builder()
+                .withNamespace(element.getNamespace())
+                .withNext(enclosingXmlEnvironment)
+                .build();
     }
 
     public XmlStringBuilder escapedElement(String name, String escapedContent) {
@@ -492,7 +500,7 @@ public class XmlStringBuilder implements Appendable, CharSequence, Element {
     }
 
     public XmlStringBuilder append(Collection<? extends Element> elements) {
-        return append(elements, null);
+        return append(elements, effectiveXmlEnvironment);
     }
 
     public XmlStringBuilder append(Collection<? extends Element> elements, XmlEnvironment enclosingXmlEnvironment) {

smack-extensions/src/main/java/org/jivesoftware/smackx/mediaelement/element/MediaElement.java

@@ -87,7 +87,7 @@ public class MediaElement implements FormFieldChildElement {
             .optAttribute("width", width)
             .rightAngleBracket();
 
-        xml.append(uris, xmlEnvironment);
+        xml.append(uris);
 
         xml.closeElement(this);
         return xml;

smack-extensions/src/test/java/org/jivesoftware/smackx/mediaelement/element/MediaElementTest.java

@@ -0,0 +1,42 @@
+/**
+ *
+ * Copyright 2019 Florian Schmaus
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.smackx.mediaelement.element;
+
+import static org.junit.Assert.assertEquals;
+
+import java.net.URI;
+import java.net.URISyntaxException;
+
+import org.junit.jupiter.api.Test;
+
+public class MediaElementTest {
+
+    @Test
+    public void simpleToXmlTest() throws URISyntaxException {
+        MediaElement.Uri uri = new MediaElement.Uri(new URI("http://example.org"), "test-type");
+
+        MediaElement mediaElement = MediaElement.builder()
+                .addUri(uri)
+                .setHeightAndWidth(16, 16)
+                .build();
+
+        String xml = mediaElement.toXML().toString();
+
+        String expected = "<media xmlns='urn:xmpp:media-element' height='16' width='16'><uri type='test-type'>http://example.org</uri></media>";
+        assertEquals(expected, xml);
+    }
+}