clean up searchClass and createClass

[git-p4: depot-paths = "//depot/": change = 1350]

DIDLLite.py

@@ -394,6 +394,16 @@ class Container(Object, list):
 			self.updateID = (self.updateID + 1) % (1l << 32)
 			Container.didUpdate(self.cd['0'])
 
+	def _addSet(self, e, items):
+		if items is not None:
+			if not isinstance(items, (list, tuple)):
+				items = [ items ]
+			for i in items:
+				el = SubElement(root, e)
+				el.text = i
+				# XXX - how to specify?
+				el.attrib['includeDerived'] = '1'
+		
 	def toElement(self):
 
 		root = Object.toElement(self)
@@ -403,14 +413,8 @@ class Container(Object, list):
 		if self.childCount:
 			root.attrib['childCount'] = str(self.childCount)
 
-		if self.createClass is not None:
-			SubElement(root, 'upnp:createclass').text = self.createClass
-
-		if self.searchClass is not None:
-			if not isinstance(self.searchClass, (list, tuple)):
-				self.searchClass = ['searchClass']
-			for i in searchClass:
-				SubElement(root, 'upnp:searchclass').text = i
+		self._addSet('upnp:createclass', self.createClass)
+		self._addSet('upnp:searchclass', self.searchClass)
 
 		if self.searchable is not None:
 			root.attrib['searchable'] = str(self.searchable)