title: Unit Testing ReactiveCocoa SignalProducer keeps coming back interrupted
body: <p>Ive been unit testing ReactiveCocoa Signals and Producers using XCTest expectations for a project im working on. For one some reason (I suspect after a pod update) my tests started failing and SignalProducer events are returning the interrupted value and never actually sending the successful value. If I set breakpoints everything seems to work fine in the actual task its just when it gets to the observer lifecycle where i dont see the value passed back.
  Using XCode 9.4.1
  ReactiveCocoa (8.0.0)
  ReactiveSwift (4.0.0)
  Result (4.0.0)</p>
  
  <pre><code>let signalExpectation = self.expectation(description: "Signal Expectation")   
  let aSut =  SignalProducer&lt;Data,WXClientError&gt; { observer, _ in
              let task = URLSession.shared.dataTask(with: url!) { (data, URLResponse, Error) in   if let Error = Error {
                      observer.send(error: WXClientError.webFailure(description: Error.localizedDescription))
                  } else if let httpResponse = URLResponse as? HTTPURLResponse {
                      if httpResponse.statusCode == 200 {
                          observer.send(value:data!)
                      }
                  }
              }
              task.resume()
          }
  
          let testObserver = Signal&lt;Data,WXClientError&gt;.Observer{ (event) in
              switch event {
              case let .value(v):
                  print("Test Observation value = \(v)")
                  signalExpectation.fulfill()
              case let .failed(error):
                  print("Test Observation error = \(error)")
              case .completed:
                  print("Test Observation completed")
              case .interrupted:
                  print("Test Observation interrupted")
              }
          }
          aSut.start(testObserver)  
          self.waitForExpectations(timeout: 5.0, handler:nil)
  </code></pre>