Memory leak when setting the default user agent

Hello,

This proposed change fixes a memory leak when setting the default PycURL user agent.

In libcurl's setstropt function (https://github.com/bagder/curl/blob/master/lib/url.c#L290), called by Curl_setopt, the s argument is copied by strdup. Since s is never freed, except on an error condition, it will be leaked.

Thank you.