Use StandardExtensionElement in JingleProvider

and some further minor jingle fixes:
- deprecate getJingleTransport() in favor of getTransport()
- Jingle.Builder now checks if the session ID is not empty

smack-core/src/test/java/org/jivesoftware/smack/parsing/StandardExtensionElementParserTest.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2015 Florian Schmaus.
+ * Copyright 2015-2017 Florian Schmaus.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -46,6 +46,9 @@ public class StandardExtensionElementParserTest {
         assertEquals("attr2-value", barNs2Element.getAttributeValue("attr2"));
 
         assertEquals("another-element-text", parsedElement.getFirstElement("another-element").getText());
+
+        String parsedElementString = parsedElement.toXML().toString();
+        assertEquals(elementString, parsedElementString);
     }
 
     @Test