Merge pull request #2052 from davidmoten/make-class-static

OperatorDoOnRequest.ParentSubscriber should be static class

src/main/java/rx/internal/operators/OperatorDoOnRequest.java

@@ -52,7 +52,7 @@ public void request(long n) {
         return parent;
     }
 
-    private final class ParentSubscriber<T> extends Subscriber<T> {
+    private static final class ParentSubscriber<T> extends Subscriber<T> {
         private final Subscriber<? super T> child;
 
         private ParentSubscriber(Subscriber<? super T> child) {