AkamaiRSpec::Response should check that @response is a response

[beegibson]

initialize should check that response is actually a RestClient::Response or Net::HTTPResponse before setting @response. headers should be explicit that the second case introduced in PR 14 is for Net:HTTPResponse. When I tried to do this quickly while reviewing PR 14 it broke a bunch of tests, so I don't have the time to deal with it at the moment.