Add function call bang test.

tests/functions.rs

@@ -143,3 +143,26 @@ fn test_functions_global_module() -> Result<(), Box<EvalAltResult>> {
 
     Ok(())
 }
+
+#[test]
+fn test_functions_bang() -> Result<(), Box<EvalAltResult>> {
+    let engine = Engine::new();
+
+    assert_eq!(
+        engine.eval::<INT>(
+            "
+                fn foo() {
+                    hello + bar
+                }
+
+                let hello = 42;
+                let bar = 123;
+
+                foo!()
+            ",
+        )?,
+        165
+    );
+
+    Ok(())
+}