read_resource should validate JSON before returning response

[josephruscio]

something like:

    def assert_valid_json(response, msg = nil)
      full_message = build_message(msg, "response should be valid JSON")
      assert_block(full_message) do
        parsable = begin
                     JSON.parse last_response.body
                     true
                   rescue JSON::ParserError
                     false
                   end

        parsable && last_response.header['Content-Type'].include?("application/json")
      end
    end