Fix castling through check

Fix castling through check

Castling through check, like walking into check, is supposed to be  tested
through early move generation in the reply node. But this early generation
was triggered by testing whether the 'to-piece' was a King, while the way
castling is implemented promotes the King to a Rook. This way castling
escaped the test. Now early move generation is done based on the from-piece.