Allow triggerRequest and signalCancel to be coalesced (#462)

See #453 and #441 (comment)

Author: jroper

tck/src/main/java/org/reactivestreams/tck/SubscriberWhiteboxVerification.java

@@ -266,8 +266,9 @@ public void required_spec208_mustBePreparedToReceiveOnNextSignalsAfterHavingCall
       @Override
       public void run(WhiteboxTestStage stage) throws InterruptedException {
         stage.puppet().triggerRequest(1);
-        stage.puppet().signalCancel();
         stage.expectRequest();
+        stage.puppet().signalCancel();
+        stage.expectCancelling();
         stage.signalNext();
 
         stage.puppet().triggerRequest(1);
@@ -812,11 +813,17 @@ public interface SubscriberPuppet {
      * Before sending any element to the subscriber, the TCK must wait for the subscriber to request that element, and
      * must be prepared for the subscriber to only request one element at a time, it is not enough for the TCK to
      * simply invoke this method before sending elements.
+     * <p>
+     * An invocation of {@link #signalCancel()} may be coalesced into any elements that have not yet been requested,
+     * such that only a cancel signal is emitted.
      */
     void triggerRequest(long elements);
 
     /**
-     * Trigger {@code cancel()} on your {@link Subscriber}
+     * Trigger {@code cancel()} on your {@link Subscriber}.
+     * <p/>
+     * An invocation of this method may be coalesced into any outstanding requests, as requested by
+     * {@link #triggerRequest(long)}, such that only a cancel signal is emitted.
      */
     void signalCancel();
   }