Fix bug in Channel.firstClosingFee

* when estimating fee for first closing tx, it was creating
  WitScript by `Scirpt(ourWitnesses).ToWitScript()` which was trying to
  deserialize script from literal sequence of bytes instead of treating
  ourWitnesses as a data which has to get pushed into stack as a wintess
  item, thus causing argument exception in `ToWitScript()`

src/DotNetLightning.Core/Channel/Channel.fs

@@ -73,8 +73,8 @@ module Channel =
                 let! dummyClosingTx = Transactions.makeClosingTx cm.FundingScriptCoin localSpk remoteSpk cm.LocalParams.IsFunder Money.Zero Money.Zero cm.LocalCommit.Spec n
                 let tx = dummyClosingTx.Value.GetGlobalTransaction()
                 tx.Inputs.[0].WitScript <-
-                    let witness = seq [ dummySig.ToBytes(); dummySig.ToBytes(); dummyClosingTx.Value.Inputs.[0].WitnessScript.ToBytes() ] |> Array.concat
-                    Script(witness).ToWitScript()
+                    let witness = seq [ dummySig.ToBytes(); dummySig.ToBytes(); dummyClosingTx.Value.Inputs.[0].WitnessScript.ToBytes() ]
+                    WitScript(witness)
                 let feeRatePerKw = FeeRatePerKw.Max (feeEst.GetEstSatPer1000Weight(ConfirmationTarget.HighPriority), cm.LocalCommit.Spec.FeeRatePerKw)
                 let vsize = tx.GetVirtualSize()
                 return feeRatePerKw.ToFee(uint64 vsize)