searchSingleIndex doesn't support offset,length pagination query

When passing offset,length parameter into searchSingleIndex ,the response have different data then what decoder expect causing error.
```
I/flutter (11302): CheckedFromJsonException
I/flutter (11302): Could not create `SearchResponse`.
I/flutter (11302): There is a problem with "hitsPerPage".
I/flutter (11302): type 'Null' is not a subtype of type 'int' in type cast
I/flutter (11302): #0 $checkedConvert (package:json_annotation/src/checked_helpers.dart:91:5)
I/flutter (11302): #1 $checkedCreate.checkedConvert (package:json_annotation/src/checked_helpers.dart:28:7)
I/flutter (11302): #2 _$SearchResponseFromJson. (package:algolia_client_search/src/model/search_response.g.dart:41:39)
I/flutter (11302): #3 $checkedCreate. (package:json_annotation/src/checked_helpers.dart:33:22)
I/flutter (11302): #4 $checkedNew (package:json_annotation/src/checked_helpers.dart:51:23)
I/flutter (11302): #5 $checkedCreate (package:json_annotation/src/checked_helpers.dart:30:10)
I/flutter (11302): #6 _$SearchResponseFromJson (package:algolia_client_search/src/model/search_response.g.dart:10:5)
I/flutter (11302): #7 new SearchResponse.fromJson (package:algolia_client_search/src/model/search_response.dart:234:7)
I/flutter (11302): #8 deserialize (package:algolia_client_search/src/deserialize.dart:454:29)
I/flutter (11302): #9 SearchClient.searchSingleIndex (package:algolia_client_search/src/api/search_client.dart:1987:12)
I/flutter (11302):
```

Start in lib/api/search_client.dart at SearchClient.searchSingleIndex, then inspect lib/model/search_response.dart, its generated search_response.g.dart decoder, and lib/deserialize.dart. Reproduce a searchSingleIndex request with offset and length, compare the returned data with the decoder's expectations, and confirm that the response is decoded without the reported hitsPerPage error.

Written by the indexing model from the issue text.