When the a request was not stubbed, the `nil` value of the webmock response was
being cached in the `webmock_responses` hash. Then, when a second request was
made with the same HTTPClient instance and an identical signature, it was not
checking for a stub again, even though there may have been one for the second request (e.g. when using a global stub hook or if another stub is registered between the 1st and 2nd requests).