Rename filled? to filled_at? as it is more intuitive
Do not make rows named argument when predefining RubiesBoard

When I predefined RubiesBoard class so I can test the AI I shouldn't
go that far and make rows named argument as it breaks the boards test

Of course I can predefine the make_board method there but it wont be
the correct decision as this is not the core functionality coming out
from the engine, and we must predefine the core only accidently