Bug fixing: proportional missing strategy

- There's a singular case when the prediction is given by a node with a
  single instance. The case must be treated separately

- Also, the path returned by the function is modified so that it
  contains the path to the first split where a missing is found. Then it
  can be returned by the model.predict method when add_path is True and
  its meaning is the longest unique path to the prediction subtree.
Adding tests for proportional missing strategy predictions