Allow NULL as value for CURLOPT_CUSTOMREQUEST option.

This fixes [bug 66109](https://bugs.php.net/bug.php?id=66109) and addresses [this so 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.

The following string options are nullable as well:

    CURLOPT_FTPPORT
    CURLOPT_FTP_ACCOUNT
    CURLOPT_RTSP_SESSION_ID
    CURLOPT_RANGE
    CURLOPT_KRBLEVEL

And for 7.33 onwards, these may be added as well:

    CURLOPT_DNS_INTERFACE
    CURLOPT_DNS_LOCAL_IP4
    CURLOPT_DNS_LOCAL_IP6