Fix TLS events

Event monitoring is not working under TLS because it isn't using Client.HTTPClient. This patch fixes it.

Includes a test using TLS mutual authentication. Probably needs some negative testing (e.g connecting HTTP client to HTTPS server or vice versa) to better validate error handling.
@rafecolton in this case, the user could use https://<host>:<port> as the endpoint.

@md5 could you change the code to use net.SplitHostPort instead of strings.HasSuffix? The change looks good otherwise.

Thank you for working on it!