Unsubscribe check on add.

Author: akarnokd

src/main/java/rx/internal/util/SubscriptionList.java

@@ -60,6 +60,9 @@ public boolean isUnsubscribed() {
      *          the {@link Subscription} to add
      */
     public void add(final Subscription s) {
+        if (s.isUnsubscribed()) {
+            return;
+        }
         if (!unsubscribed) {
             synchronized (this) {
                 if (!unsubscribed) {