fix call for immutable context, add it to test

asg/src/expression/call.rs

@@ -71,7 +71,7 @@ impl<'a> ExpressionNode<'a> for CallExpression<'a> {
     }
 
     fn is_mut_ref(&self) -> bool {
-        false
+        true
     }
 
     fn const_value(&self) -> Option<ConstValue> {

compiler/tests/circuits/mutable_call_immutable_context.leo

@@ -3,10 +3,23 @@ circuit TestMe {
 
     function test_me(mut self) -> u8 {
         self.x += 1;
-        return self.x
+        return self.x;
     }
+
+    function new() -> Self {
+        return Self { x: 1u8 };
+    }
+}
+
+function my_fn() -> TestMe {
+  return TestMe { x: 0u8 };
 }
 
 function main () {
     const t = TestMe {x: 6u8}.test_me();
+    console.assert(t == 7u8);
+    const u = my_fn().test_me();
+    console.assert(u == 1u8);
+    const v = TestMe::new().test_me();
+    console.assert(v == 2u8);
 }