Serialize attributes from any class inherits from AttributeContainer

In the current version, SoapSerializationEnvelope class will serialize attributes only from SoapObject class. The second way of using ksoap2 is to create a custom class which implements KvmSerializable interface. But in our generator (http://easywsdl.com) we sometimes create custom class (with KvmSerializable) and we inherits from AttributeContainer (because some properties must be serialize as attributes). But in the current version of ksoap2 these attributes will not be serialized. 
My refactor allows to serialize attributes from any class which inherits from AttributeContainer