message: fix gcs compose mode
diff: @@ -487,7 +487,7 @@ GCSFileSystemImplementation::GCSFileSystemImplementation(
  google::cloud::storage::Client&& gcs_client)
  : gcs_client(gcs_client), block_cache_lock() {
  const char* append_mode = std::getenv(kAppendMode);
  - compose = (append_mode != nullptr) && (!strcmp(kAppendMode, append_mode));
  + compose = (append_mode != nullptr) && (!strcmp(kComposeAppend, append_mode));
  uint64_t value;
  block_size = kDefaultBlockSize;
  