Fixing problem with rating change not being equal

[wakeless]

There's a problem with matches where they don't necessarily have the same change (with 2 players the same rating)

This breaks other specs though and I'm not sure how the expectations were generated.

[hellola]

This is actually a considerable problem as:

x = Elo::Player.new
y = Elo::Player.new

500.times do
  x.wins_from(y)
  y.wins_from(x)
end

puts x.rating
puts y.rating

results in around 500 for each whereas with this fix its 994 and 1001