Fix serialization of transaction_paymentInfo (#4146)

* Add serialization test for u128

* Fix crash on serde of u128

substrate/primitives/sr-primitives/src/weights.rs

@@ -95,6 +95,7 @@ impl<BlockNumber: Copy> WeighBlock<BlockNumber> for SingleModule {
 /// A generalized group of dispatch types. This is only distinguishing normal, user-triggered transactions
 /// (`Normal`) and anything beyond which serves a higher purpose to the system (`Operational`).
 #[cfg_attr(feature = "std", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "std", serde(rename_all = "camelCase"))]
 #[derive(PartialEq, Eq, Clone, Copy, Encode, Decode, RuntimeDebug)]
 pub enum DispatchClass {
 	/// A normal dispatch.