ClientOptions.Transport should be of type http.RoundTripper

[flimzy]

At present, ClientOptions contains the following field:

    // An optional pointer to `http.Transport` that will be used with a default HTTPTransport.
    // Using your own transport will make HTTPProxy, HTTPSProxy and CaCerts options ignored.
    HTTPTransport *http.Transport

*http.Transport is meant to be used as an implementation of the http.RoundTripper interface.

Updating all instances of *http.Transport to http.RoundTripper seems to work perfectly, so this should not even be a breaking change.