CDRIVER-3734 discovered that sending OCSP requests with OpenSSL's OCSP_sendreq_bio may not set the Host header. Some responders, (http://ocsp.sca1b.amazontrust.com in particular) consider it a malformed HTTP request and return a 400 response.

I think it would be beneficial to add an assertion to the mock responder to assert that the Host header is present. Drivers will benefit from this additional test coverage since the mock responder is synced to https://github.com/mongodb-labs/drivers-evergreen-tools/blob/master/.evergreen/ocsp/mock_ocsp_responder.py