Adds some light testing for Rex's HTTP client lib

In light ofi PR #1476, it would be nice to have some basic, modern,
maintained testing on Rex's HTTP Client proto library.

My rspec fu is quite weak, of course, but this should cover the very
basic cases. There are lots of pending holes, but hey, it's a start.
Actually run this spec.
Let's not intro functionality as testing

That's a bad habit to get into.
Move pending messages to it()'s args
Pending hashes need to end w a block
Backed out all the fails from the auth bits
Moving @cli and @ip to instance vars
Merge branch 'master' into rspec/rex-http-client
Merge branch 'rspec/rex-http-client'

This lands https://github.com/rapid7/metasploit-framework/pull/1478

Talked to @jlee-r7 on Friday, and since it's only rspec tests, we agreed
that there shouldn't be anything stopping people from landing their own
tests -- we want to encourage rspec writing as much as we can. If you
can write passing tests, then by all means, land them yourself.