Exception raised for "None" probability after call to set_prob_type

[Pierre Denis]

By default, None probability is converted to the right value:

lea.pmf({"Head": '1/4', "Tail": None})
# -> Head : 1/4
#    Tail : 3/4

But this breaks after calling set_prob_type with an argument different from "x":

lea.set_prob_type("r")
lea.pmf({"Head": '1/4', "Tail": None})
# -> TypeError: argument should be a string or a Rational instance

[Pierre Denis]

Fix issue with None probability (refs #86)

→ <<cset 155fc15e4b57>>

[Pierre Denis]

Fixed on Lea 4.