title: ReactiveCocoa - flattenMap block never called
body: <p>I have a problem using <code>flattenMap</code> method of <code>RACSignal</code> - the block never gets called. If I <code>subscribeNext</code> to the same signal, it works just fine, the problem is only with <code>flattenMap</code>.   </p>
  
  <p>Here's what works fine  </p>
  
  <pre><code>[[self.aSignal combineLatestWith:self.otherSignal] subscribeNext:^(RACTuple *tuple) {
      // gets called just fine
  }];
  </code></pre>
  
  <p>And here's what doesn't work:  </p>
  
  <pre><code>self.yetAnotherSignal = [[self.aSignal combineLatestWith:self.otherSignal] flattenMap:^RACStream *(RACTuple *tuple) {
      // never gets called
      return returnSignal;
  }];
  </code></pre>
  
  <p>Am I missing something? Or do I misunderstand how <code>flattenMap</code> works?</p>