title: Watson Assistant: Context variable in slot is not being updated
body: <p>I'm currently working with some required slots in Watson Assistant, but I have a problem that I don't understand.
  When the user enters the node with the slot the first time, the variable is updated with the corresponding value, but the second time I try to enter the node (without entering the entity), it ignores the slot as if the entity had been entered.
  I know that the context variable is keeping the original value, but should not verify that the entity exists instead of the variable?. Are there some best practices to clean this kind of context variables (either in the UI tool or the backend)?</p>