Position.State.enpasant should be nil, if no en passant is available

[christian-heise]

At every pawn move the possible en passant square is written to Game.Position.State.enpasant. However, when I want to compare two Position variables, if they are equal, this leads to wrong results. The positions after 1. e4 e5 2. d4 d5 and 1. d4 d5 2. e4 e5 should be exactly the same, however the Position variable is not equal because the respective last move was a different pawn move, even though no en passant is available.