[2.0] Update setHttpClient signature to hint client interface

[bencorlett]

On a side note, I'm almost deterred by suffixing Interface - I'd rather have the interface readable and implementations describe exactly what their concrete implementation does different... Thoughts?

[stevenmaguire]

Yeah, I'm usually in favor of that approach as well. This case was a bit different as the class still needs to know about the concrete implementation and the interface, both of which are provided by the guzzle project.

Perhaps we use HttpClient for the interface and something like GuzzleClient for the implementation references? I am deterred by Client begin reserved for third-party packages :)