Cache update with optimisticResponse

[awaik]

Description

During diving into the code for #18, I found some problems with the cache update with optimisticResponse.

If we make the mutation with Optimistic Response

final mutation = ProfileBmiMutation2(
  buildVars: (b) => b..input = profileInput,
  updateCacheHandlerKey: IdCacheHandlers.profileBmiMutation,
  optimisticResponse: optimisticRequest,
).copyWith(
  context: Context.fromList(
    [
      HttpLinkHeaders(
        headers: idToken,
      ),
    ],
  ),
);
client
    .responseStream(mutation)
    .firstWhere((response) => response.optimistic)
    .then((response) {});

cache updates incorrectly.

Assumption where the bug is

Maybe it is connected with code here /lib/src/cache/cache_proxy.dart, because if we change optimistic: _optimistic, to optimistic: false, it starts to work correctly (in this exact use-case).
I guess, there is more logic. I just pointed out what I've found.

  void writeQuery(
    Request request,
    Map<String, dynamic> data,
  ) =>
      _cache.writeQuery(
        request,
        data,
        optimistic: false,
        queryId: _queryId,
      );

Screenshot

1. When we make the first request cache is not updated.
2. But if we open the screen again, cache updates.
3. After that, other updates do not change the cache.

[smkhalsa]

@awaik can you add a failing test for this, and I'll take a look?

[awaik]

@smkhalsa ok, I do it after finishing the example with mutations