Implement of RateLimiter.setRate can be simplified

[xianwdong]

public final void setRate(double permitsPerSecond) {
    checkArgument(
        permitsPerSecond > 0.0 && !Double.isNaN(permitsPerSecond), "rate must be positive");
    synchronized (mutex()) {
      doSetRate(permitsPerSecond, stopwatch.readMicros());
    }
  }

when permitsPerSecond is NaN，NaN > 0.0 return false, so check "!Double.isNaN(permitsPerSecond)" is redundant