can extend rails over air with piston

relates to:

https://mojang.atlassian.net/browse/MC-1677

https://mojang.atlassian.net/browse/MC-5726

If rails are being pushed off a ledge, by player spamming rails onto the first free block infront of the retracted phase of a fast firing piston, (either a piston, or a block it is pushing), they are able to extend out over air (2013-03-08_22.32.39.png), but are not functional, minecarts fall through them as if the track weren't present (all rail types tried.) Occasionally, the first rail over an air block will break but keep pushing other sections out (2013-03-08_22.39.04.png).