Ensure uppercase request method for Curb

Curb's `http` method takes its argument and sets it as the value for libcurl's `CURLOPT_CUSTOMREQUEST`. Apparently this *needs* to be uppercase, though we're not sure why it's failing now (or how it *ever* worked, really). This fixes [the failing tests](https://travis-ci.org/bblimke/webmock/jobs/10469034) on our local machine.