Speed modifiers have no effect when the player is not on the ground

Speed modifiers (i.e. potion effects, sprinting, etc.) have no effect on the player if they are not touching the ground. This includes flying, swimming, sprint-jumping, etc.

Several drawbacks to this:
1) sprint-jumping with a speed potion actually slows you down
2) slowness potions have nearly no effect if the player is sprint-jumping
3) speed potions and slowness potions both have absolutely no effect when the player is swimming
4) players cannot use speed potions to fly faster in creative mode

Please fix this? It seems like it should be easy enough...