ssl_version was only being set if SSL auth settings were present

Pull request for https://github.com/savonrb/httpi/issues/106 .

I have changed the conditions that surround that method so that the SSL version is set regardless of SSL auth settings. I'm not entirely happy with the way that the SSL version stuff is wrapped up in the `@request.auth` object, and the use of both `@request.auth.ssl?` and `@request.ssl?` but I wanted to create a minimal pull request that solved the problem.

I had to rejig the test-suite so that the SSL version tests weren't encapsulated in a context where the SSL auth settings were getting configured. New tests now pass with my modifications to the following adapters:

- net_http
- httpclient
- curb

I'm not a user of RSpec, but I think I managed to write effective tests by inferring from what was already there, however the net_http adapter spec is quite different to the others and I wasn't able to write a test for that adapter, but have applied the same fix.

em_http and rack don't seem to support ssl (or at least ssl client auth? not sure ...) so I didn't change them. excon has a bunch of ssl stuff but no mention of ssl_version ..