title: dateFromString always returns null with dateformatter
body: <p>I have the following code:</p>
  
  <pre><code> NSDateFormatter * df = [[NSDateFormatter alloc] init];
      [df setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
      [df setTimeZone:[NSTimeZone systemTimeZone]];
      [df setFormatterBehavior:NSDateFormatterBehaviorDefault];
  
      [df dateFromString:@"2011-04-12 10:00:00"];
  </code></pre>
  
  <p>In which it always generates a null date. Why is this?</p>