Add assertion for mirrored state in mixed online/offline request

Such a request should never occur when the index is not mirrored.

Author: Clément Le Provost

algoliasearch/src/offline/java/com/algolia/search/saas/MirroredIndex.java

@@ -647,6 +647,9 @@ private abstract class OnlineOfflineRequest implements Request {
          * Construct a new mixed online/offline request.
          */
         public OnlineOfflineRequest(@NonNull CompletionHandler completionHandler) {
+            if (!mirrored) {
+                throw new IllegalStateException("This index is not mirrored");
+            }
             this.completionHandler = completionHandler;
         }
 
@@ -678,7 +681,7 @@ public OnlineOfflineRequest start() {
             // Since most methods use the main thread for callbacks, we have to use it as well.
 
             // If the strategy is "offline only", well, go offline straight away.
-            if (requestStrategy == Strategy.OFFLINE_ONLY && mirrored) {
+            if (requestStrategy == Strategy.OFFLINE_ONLY) {
                 startOffline();
             }
             // Otherwise, always launch an online request.