fix contractSize bug
fix contractSize bug

contractSize did not take into account encryption overhead, causing
contracts to be too small.
There is a bigger question of whether f.pieceSize should report the
size pre- or post-encryption. I think there's an argument to be made
for the latter, since the download logic has to tiptoe around it as
well.

ChrisSchinnerl/Sia,johnathanhowell/Sia,Mingling94/Sia,MrStrong/Sia,NebulousLabs/Sia,mharkus/Sia,mtlynch/Sia,mnsl/Sia,mtlynch/Sia,ChrisSchinnerl/Sia,ericflo/Sia,rivine/rivine,NebulousLabs/Sia,ericflo/Sia,mharkus/Sia,MrStrong/Sia,rivine/rivine,mnsl/Sia