sweep: always send signed output's pk script

Fixes #457.
The SignOutputRaw RPC is normally too lenient when it comes to fields
not being set, which is why this used to work previously. But with lnd
in remote signing mode, we need all the extra meta information available
on an input being signed for, so we actually need the pk script being
set.
Loop fully works with a remote signing lnd backend pair after applying
this fix.

sweep/sweeper.go

@@ -61,7 +61,8 @@ func (s *Sweeper) CreateSweepTx(
 	signDesc := lndclient.SignDescriptor{
 		WitnessScript: htlc.Script(),
 		Output: &wire.TxOut{
-			Value: int64(amount),
+			Value:    int64(amount),
+			PkScript: htlc.PkScript,
 		},
 		HashType:   txscript.SigHashAll,
 		InputIndex: 0,