revert change to onStart request

Author: davidmoten

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

@@ -110,8 +110,9 @@ public void call() {
 
         @Override
         public void onStart() {
-            // no need for more than 1 at a time
-            request(1);
+            // no need for more than 1 at a time since we concat 1 at a time, so we'll request 2 to start ...
+            // 1 to be subscribed to, 1 in the queue, then we'll keep requesting 1 at a time after that
+            request(2);
         }
 
         private void requestFromChild(long n) {