Add test to make sure call_fn does not pollute scope.

tests/call_fn.rs

@@ -38,6 +38,10 @@ fn test_call_fn() -> Result<(), Box<EvalAltResult>> {
             fn hello() {
                 41 + foo
             }
+            fn define_var() {
+                let bar = 21;
+                bar * 2
+            }
         ",
     )?;
 
@@ -50,6 +54,11 @@ fn test_call_fn() -> Result<(), Box<EvalAltResult>> {
     let r: INT = engine.call_fn(&mut scope, &ast, "hello", ())?;
     assert_eq!(r, 42);
 
+    let r: INT = engine.call_fn(&mut scope, &ast, "define_var", ())?;
+    assert_eq!(r, 42);
+
+    assert!(!scope.contains("bar"));
+
     assert_eq!(
         scope
             .get_value::<INT>("foo")