use strings as values, with option to allow removing

tests/operation_transform_invariant.rs

@@ -20,7 +20,7 @@ fn operation_strategy() -> impl Strategy<Value = Operation> {
             Operation::Update {
                 uuid,
                 property,
-                value: true.into(),
+                value: Some("true".into()),
                 timestamp: Utc::now(),
             }
         }),

tests/sync.rs

@@ -18,7 +18,7 @@ fn test_sync() {
     db1.apply(Operation::Update {
         uuid: uuid1,
         property: "title".into(),
-        value: "my first task".into(),
+        value: Some("my first task".into()),
         timestamp: Utc::now(),
     });
 
@@ -27,7 +27,7 @@ fn test_sync() {
     db2.apply(Operation::Update {
         uuid: uuid2,
         property: "title".into(),
-        value: "my second task".into(),
+        value: Some("my second task".into()),
         timestamp: Utc::now(),
     });
 
@@ -41,13 +41,13 @@ fn test_sync() {
     db1.apply(Operation::Update {
         uuid: uuid2,
         property: "priority".into(),
-        value: "H".into(),
+        value: Some("H".into()),
         timestamp: Utc::now(),
     });
     db2.apply(Operation::Update {
         uuid: uuid2,
         property: "project".into(),
-        value: "personal".into(),
+        value: Some("personal".into()),
         timestamp: Utc::now(),
     });