title: Filling NSMutableData with bytes results in zero length
body: <p>When setting up NSMutableData like this:</p>
  
  <pre><code>NSMutableData* mRgb = [NSMutableData dataWithCapacity:3];
  ((char*)[mRgb mutableBytes])[0] = 10;
  ((char*)[mRgb mutableBytes])[1] = 90;
  ((char*)[mRgb mutableBytes])[2] = 160;
  </code></pre>
  
  <p>I have the problem that the length is still 0:</p>
  
  <pre><code>int len = [mRgb length]; // Is 0!
  </code></pre>
  
  <p>Why is that so?</p>