fix a void test

[ruslandoga]

This test case was missing an assert and was passing without testing what it was supposed to. After adding assert it doesn't pass because in :async or :none modes maybe_log_result's result is

{:ok,
 %Task{
   owner: #PID<0.393.0>,
   pid: #PID<0.406.0>,
   ref: #Reference<0.1323136707.3469803524.187842>
 }}

That makes me think {:error, {:request_failure, last_error}} is unreachable under default config and client errors would never be logged. Maybe instead of logging on task / result, logging for invalid request could happen from within the task?

[ruslandoga]

closing in favor of #517