HTTPS support: Warn user of mismatched combinations

[okin]

From @vidartf's comment in #50:

The only possible improvement I can see would be to give a warning if a user requests a mismatched secure/non-secure server/client/port tuple. Could be an easy typo that would be hard to catch, but I'm not sure if it is worth the effort though, so I will leave that as a possible follow-up PR (ping if you want it).

Basically we should catch combinations of secure/unsecure fixtures and warn users in this case.