fix hang at end of PUT to proxied p2p http remote

fix hang at end of PUT to proxied p2p http remote

sendExactly will now be sure to evaluate the whole lazy ByteString.

In this case, the lazy ByteString was exactly the right lenth.
But, it seems that L.take caused it to not actually be fully evaluated.

In servePut, this manifested as gather never being fully evaluated,
which caused the hang.

Very, very subtle, and horrible bug. Clearly the use of lazy ByteString
(or really just laziness) is at fault, and it would be very worth moving
to conduit or whatever to avoid this.