Use unmodifiableMap in the constructor

Author: Andrey Ershov

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/get/GetSnapshotsResponse.java

@@ -107,6 +107,8 @@ public GetSnapshotsResponse(Collection<Response> responses) {
                 this.failedResponses.put(response.repository, response.error);
             }
         }
+        successfulResponses = Collections.unmodifiableMap(successfulResponses);
+        failedResponses = Collections.unmodifiableMap(failedResponses);
     }
 
     public GetSnapshotsResponse() {
@@ -142,14 +144,14 @@ public Set<String> getRepositories() {
      * Returns a map of repository name to the list of {@link SnapshotInfo} for each successful response.
      */
     public Map<String, List<SnapshotInfo>> getSuccessfulResponses() {
-        return Map.copyOf(successfulResponses);
+        return successfulResponses;
     }
 
     /**
      * Returns a map of repository name to {@link ElasticsearchException} for each unsuccessful response.
      */
     public Map<String, ElasticsearchException> getFailedResponses() {
-        return Map.copyOf(failedResponses);
+        return failedResponses;
     }
 
     /**