fix(callable): __call cannot be in a nested metatable

lua/pl/types.lua

@@ -11,7 +11,14 @@ local types = {}
 --- is the object either a function or a callable object?.
 -- @param obj Object to check.
 function types.is_callable (obj)
-    return type(obj) == 'function' or getmetatable(obj) and getmetatable(obj).__call and true
+    if type(obj) == 'function' then
+        return true
+    end
+    local mt = getmetatable(obj)
+    if not mt then
+        return false
+    end
+    return type(rawget(mt, "__call")) == "function"
 end
 
 --- is the object of the specified type?.