Use native php function to handle autoloader callables

[nervo]

Fix #673

This commit https://github.com/Luracast/Restler/commit/b3ded4194057951ac96a41104f76b4b2eed62cc7 introduced a weird way to handle autoloader callables, where a simple native php function call_user_func can do the trick.

If approved and merged, would you consider backport it to 3 and 4 branches ?

Thx 😗