Allow NULL as value for CURLOPT_CUSTOMREQUEST option.

This fixes [bug 66109](https://bugs.php.net/bug.php?id=66109) and addresses [this question](http://stackoverflow.com/questions/4163865/how-to-reset-curlopt-customrequest).

It allows for a NULL value to be passed for CURLOPT_CUSTOMREQUEST, so that the internal behaviour can be reset.