fix test and add some javadoc

Author: aozarov

src/main/java/com/google/gcloud/storage/ListResult.java

@@ -41,10 +41,16 @@ public ListResult(NextPageFetcher<T> pageFetcher, String cursor, Iterable<T> res
     this.results = results;
   }
 
+  /**
+   * Returns the cursor for the nextPage or {@code null} if no more results.
+   */
   public String nextPageCursor() {
     return cursor;
   }
 
+  /**
+   * Returns the results of the nextPage or {@code null} if no more result.
+   */
   public ListResult<T> nextPage() {
     if (cursor == null || pageFetcher == null) {
       return null;

src/test/java/com/google/gcloud/storage/SerializationTest.java

@@ -51,7 +51,7 @@ public class SerializationTest {
       Collections.<BatchResponse.Result<Blob>>emptyList(),
       Collections.<BatchResponse.Result<Blob>>emptyList());
   private static final ListResult<Blob> LIST_RESULT =
-      new ListResult<>("c", Collections.singletonList(Blob.of("b", "n")));
+      new ListResult<>(null, "c", Collections.singletonList(Blob.of("b", "n")));
   private static StorageService.BlobListOption BLOB_LIST_OPTIONS =
       StorageService.BlobListOption.maxResults(100);
   private static StorageService.BlobSourceOption BLOB_SOURCE_OPTIONS =