Cast CURL constants to string that cause a InvalidArgumentException

[codedmonkey]

Followup on #62

After some quick testing, I noticed some inconsistencies because the Symfony source code doesn't use strict typing. So when you pass the CURL constants as array keys, it will be converted to strings by the OptionsResolver itself.

# This works
        $resolver->setDefaults(
            [
                CURLOPT_HEADER => false
            ]
        );

# This doesn't work
        $resolver->setDefault(CURLOPT_HEADER, false);

[dbu]

the reason why the array works might also be the weird way of how php handles array keys (autoconverting strings to integer when they look like they could be numeric)

anyway, the fix looks right, thanks!