Use the absolute value of the up attribute when calculating up_extent and side_extent

[Linentio]

Fixes dimforge/bevy_rapier#413

I think that we should use the absolute value of the up attribute of a KinematicCharacterController when calculating the value of the up_extent and side_extent variables.
Concerning the up_extent variable, if you make the up attribute point downward, parry will make the app crash because of the assert in the loosened function of the AABB struct since the amount of loosening would be negative.
And concerning the side_extent variable, the value would not be the same if the up attribute points upward or downward.

[sebcrozet]

Thanks!