fix: PHP 8.1 "Serialization of 'ReflectionMethod' is not allowed"

[j-bonex]

With 0925b5099919300a239909588351a6482c5e792d there have been proper fixes for CI_Migration and CI_Xmlrpcs classes. But for CI_Codeigniter the former is_callable check has been removed without any substitution.

This causes "Serialization of 'ReflectionMethod' is not allowed" errors in several cases, especially when running PHPUnit integration tests using @runInSeparateProcess Feature.

Please accept the same fix for CI_Codeigniter main class as well.