Fix incorrect merge in previous PR (mimblewimble#267)

libwallet/src/slate.rs

@@ -187,8 +187,8 @@ pub struct Slate {
 	/// TTL, the block height at which wallets
 	/// should refuse to process the transaction and unlock all
 	/// associated outputs
-	#[serde(with = "secp_ser::string_or_u64")]
-	pub ttl_cutoff_height: u64,
+	#[serde(with = "secp_ser::opt_string_or_u64")]
+	pub ttl_cutoff_height: Option<u64>,
 	/// Participant data, each participant in the transaction will
 	/// insert their public data here. For now, 0 is sender and 1
 	/// is receiver, though this will change for multi-party