make instantSeal engine backwards compatible, closes #9696 (#9700)

json/src/spec/engine.rs

@@ -26,7 +26,7 @@ pub enum Engine {
 	Null(NullEngine),
 	/// Instantly sealing engine.
 	#[serde(rename="instantSeal")]
-	InstantSeal(InstantSeal),
+	InstantSeal(Option<InstantSeal>),
 	/// Ethash engine.
 	Ethash(Ethash),
 	/// BasicAuthority engine.
@@ -71,6 +71,17 @@ mod tests {
 			_ => panic!(),
 		};
 
+		let s = r#"{
+			"instantSeal": null
+		}"#;
+
+		let deserialized: Engine = serde_json::from_str(s).unwrap();
+		match deserialized {
+			Engine::InstantSeal(_) => {},	// instant seal is unit tested in its own file.
+			_ => panic!(),
+		};
+
+
 		let s = r#"{
 			"Ethash": {
 				"params": {