fix crash when responding with a non-ground list

[McModknower]

Currently when the API wants to serialize a list that contains non-ground values, the list case is not triggered (as X or Xs is not ground), and it falls through to the case for terms. Since a non-empty list is also a term of the form '[|]'(Head, Tail), a call to rosprolog_encode([Head, Tail],Args_json) gets made. Since the Head or the Tail is not ground, the same procedure happens over and over until the stack is full.

To prevent this case, i added the ability to serialize lists that contain non-ground entries.
To make sure that the case for terms never tries to encode lists, i added a check to make sure that the Functor is not '[|]'.