[2.3] Remove possible call_user_func()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Merging this in 2.3 enhances performance a bit, but more importantly will ease future merges into 3.0.
this breaks on PHP 5.3 because of ``array($obj, $method)`` callables (which is *always* the case here btw). See the Travis output.