Make request_parameters optional (#91)

Some events e.g. s3:Replication:OperationCompletedReplication send
'null' value in request_parameters and the deserialization can fail in
such cases ('deault' doesn't help here)

Marking this field as optional will ensure that such events can also be
deserialized without any error.

src/s3/types.rs

@@ -748,7 +748,7 @@ pub struct NotificationRecord {
     #[serde(alias = "userIdentity", default)]
     pub user_identity: UserIdentity,
     #[serde(alias = "requestParameters", default)]
-    pub request_parameters: RequestParameters,
+    pub request_parameters: Option<RequestParameters>,
     #[serde(alias = "responseElements", default)]
     pub response_elements: ResponseElements,
     #[serde(alias = "s3", default)]