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-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;