# File lib/rhc/rest/httpclient.rb, line 135
        def challenge(uri, param_str = nil)
          return false if caller.any?{ |s| s =~ /webmock.*httpclient_adapter.*build_request_signature/ }
          uri = urify(uri)
          # httpclient < 2.4.0 uses @challengeable, >= 2.4.0 uses @challenge
          challenges = @challenge || @challengeable
          challenged = challenges[uri]
          challenges[uri] = true
          !challenged
        end