It's not quite clear from the curb docs, but it's possible to use Curb::Easy#http_post & Curb::Easy#http_put methods without arguments (by setting post_body or put_data beforehand). In the current webmock implementation the data argument is mandatory and this patch makes it optional as it is in curb.
Curb::Easy#http_post & Curb::Easy#http_put can be used without arguments
Thanks Eugene! Your changes are now released in version 1.6.2.