Assert we get a generation header

Author: nicktindall

modules/repository-gcs/src/main/java/org/elasticsearch/repositories/gcs/GoogleCloudStorageRetryingInputStream.java

@@ -158,6 +158,8 @@ private Long parseGenerationHeader(HttpResponse response) {
                 assert false : "Unexpected value for x-goog-generation header: " + generationHeader;
                 return null;
             }
+        } else {
+            assert false : "X-Goog-Generation header was missing?";
         }
         return null;
     }