Fail early if a null subscription is added to a CompositeSubscription.

Otherwise, it'll just fail late when unsubscribing, which is much harder
to trace.

Author: jnlopar

src/main/java/rx/subscriptions/CompositeSubscription.java

@@ -55,6 +55,9 @@ public synchronized boolean isUnsubscribed() {
      *          the {@link Subscription} to add
      */
     public void add(final Subscription s) {
+        if (s == null) {
+            throw new IllegalArgumentException("Added Subscription cannot be null.");
+        }
         Subscription unsubscribe = null;
         synchronized (this) {
             if (unsubscribed) {

src/test/java/rx/subscriptions/CompositeSubscriptionTest.java

@@ -337,4 +337,11 @@ public void testTryRemoveIfNotIn() {
 
         csub.remove(csub1); // try removing agian
     }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testAddingNullSubscriptionIllegal() {
+      CompositeSubscription csub = new CompositeSubscription();
+      csub.add(null);
+    }
+
 }