Steps to reproduce

1. Define a ConversationHandler where one if its states corresponds to another ConversationHandler
2. Trigger the nested ConversationHandler
3. End the nested ConversationHandler using the Conversation.END state.

Expected behaviour

There should be an option to manipulate the "external" conversation state, as possible with other Handlers.

Actual behaviour

Since the last return value of the nested conversation is END, there is no option to alter the external conversation state, and it's "stuck" in that state.

Linked to #385