Do not error when a property is read-only.

tests/get_set.rs

@@ -7,6 +7,7 @@ fn test_get_set() -> Result<(), Box<EvalAltResult>> {
     #[derive(Clone)]
     struct TestStruct {
         x: INT,
+        y: INT,
     }
 
     impl TestStruct {
@@ -18,8 +19,12 @@ fn test_get_set() -> Result<(), Box<EvalAltResult>> {
             self.x = new_x;
         }
 
+        fn get_y(&mut self) -> INT {
+            self.y
+        }
+
         fn new() -> Self {
-            TestStruct { x: 1 }
+            TestStruct { x: 1, y: 0 }
         }
     }
 
@@ -28,9 +33,13 @@ fn test_get_set() -> Result<(), Box<EvalAltResult>> {
     engine.register_type::<TestStruct>();
 
     engine.register_get_set("x", TestStruct::get_x, TestStruct::set_x);
+    engine.register_get("y", TestStruct::get_y);
+    engine.register_fn("add", |value: &mut INT| *value += 41);
     engine.register_fn("new_ts", TestStruct::new);
 
     assert_eq!(engine.eval::<INT>("let a = new_ts(); a.x = 500; a.x")?, 500);
+    assert_eq!(engine.eval::<INT>("let a = new_ts(); a.x.add(); a.x")?, 42);
+    assert_eq!(engine.eval::<INT>("let a = new_ts(); a.y.add(); a.y")?, 0);
 
     Ok(())
 }