title: Unable to filter RACSignal events
body: <p>My ViewController contains a UISearchBar and implements the UISearchBarDelegate protocol. I've created a signal for <code>searchBartextDidChange:</code> that fires correctly with <code>subscribeNext</code>:</p>
  
  <pre><code>RACSignal *searchTextChangeSignal = [self rac_signalForSelector:@selector(searchBar:textDidChange:) fromProtocol:@protocol(UISearchBarDelegate)];
  
  [searchTextChangeSignal subscribeNext:^(id x){
    // This works.
  }];
  </code></pre>
  
  <p>At this point, I'd like to filter the results of this filter to 1) Only include text that is greater than 3 characters, and 2) throttled by 300 ms. My attempt:</p>
  
  <pre><code>[[searchTextChangeSignal filter:^(RACTuple *tuple) {
    NSString *textEnteredIntoSearchBar = (NSString *)tuple.second;
    return textEnteredIntoSearchBar.length &gt; 3;
  }] throttle:300];```
  </code></pre>
  
  <p>The code above doesn't work. The blocks are never executed. If I replace the <code>filter</code> method with <code>subscribeNext</code>, the <code>subscribeNext</code> block <em>does</em> execute. Furthermore, XCode autocompletes the <code>filter</code> method above, so the method is available. Is there something I'm missing here? What's the correct way to do this? Any help is much appreciated.</p>