NullReferenceException if response is not configured.

[perfectsquircle]

If a request is configured, but a response is never configured, a NullReferenceException is thrown when SendAsync is called on the HTTP client. This is confusing to the user and takes some time to unravel what went wrong.

mockHttp.Expect("/foo")
        .WithQueryString("bar", "bat")
        .WithQueryString("fizz", "buzz");
        // forgot to configure response

httpClient.GetAsync("/foo"); // throws NullReferenceException.

[0x326]

I actually just ran into this myself, so I can confirm.

[FroggieFrog]

@richardszalay Is there any chance of a new release on Nuget with this fix included?

[richardszalay]

Verion 7.0.0 includes this change.