mempool: Correct cmd field for rejected txns.

This commit corrects an issue where the reject message sent in response
to rejected transactions had the Cmd field set to "block" instead of
"tx".

Fixes btcsuite#436

blockmanager.go

@@ -499,7 +499,7 @@ func (b *blockManager) handleTxMsg(tmsg *txMsg) {
 		// Convert the error into an appropriate reject message and
 		// send it.
 		code, reason := errToRejectErr(err)
-		tmsg.peer.PushRejectMsg(wire.CmdBlock, code, reason, txHash,
+		tmsg.peer.PushRejectMsg(wire.CmdTx, code, reason, txHash,
 			false)
 		return
 	}