Use correctly cased HTTP verb for DELETE requests

Currently, we set the HTTP method to `"delete"` in `Curl::Easy#delete=`, but it should actually be set to `"DELETE"` in order for it to work consistently across all conforming HTTP servers, as HTTP methods are case sensitive. See [RFC 2616 section 5.1.1](http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.1).