fix(search): add missing AbTestID property (#847)

Author: morganleroi

src/Algolia.Search/Models/Search/SearchResponse.cs

@@ -155,6 +155,11 @@ public class SearchResponse<T> where T : class
         /// </summary>
         public int AbTestVariantID { get; set; }
 
+        /// <summary>
+        ///  A/B test ID. This is only included in the response for indices that are part of an A/B test.
+        /// </summary>
+        public int? AbTestID { get; set; }
+
         /// <summary>
         ///  The query string that will be searched, after normalization.
         /// </summary>