fixed last remaining issues before v2.1.0

Hey @rubiii, here's the fixes, build success for all rubies!

The first two errors looked like a copy/paste of net_http_spec.rb as a base for supporting excon and net_http_persistent. Since there was no implementation in either adapter or respective clients, of course these failed.  I've changed the specs to pending and added comments so that contributors can implement NTLM if they want to in the future.  For this release, I think it's sufficient to require net_http for NTLM access.  What do you think?

When I originally added NTLM support to httpi, I targeted net_http as the simplest to get the idea working.  I didn't think it was necessary to support NTLM for all adapters, but I suppose it would be nice.  We might want to pull the NTLM strategy out of net_http in the future so that it can be shared across adapters.  

The 3rd error was simply a problem with namespace that @pmorton must have missed during refactoring.