Fixed false positives with magic method case

The function name sniff returns a 'Method name "Foo::__toString" is invalid; only PHP magic methods should be prefixed with a double underscore' error when parsing the following code :

    <?php

    class Foo
    {
        public function __toString()
        {
            // ...
        }
    }

This is of course a false positive, due to the fact that the parent PEAR sniff stores the magic method names in lowercase. This pull request simply fixes this behaviour, according to PEAR's parent implementation.