Allow partial overriding of fixtures

[innocenzi]

So I've been using automatic fixture generation to test integration with an API that returns a lot of data.

However, I find myself duplicating some fixtures to edit some information in them so my assertions can pass depending on what I send in the request.

Instead of doing that, it would be great to have the following API:

MockClient::global([
    UpdateDealRequest::class => MockResponse::fixture('pipedrive/deal/update-16685-response', merge: [
        'status' => 'won'
    ]),
]);

The merge parameter would merge the array into the mocked response, so the following assertion would pass:

expect($response->dto())->status->toBe(Status::WON);

I am totally open to PRing that, I created the issue to avoid working on it if it was not going to be accepted 👍

[innocenzi]

Ended up PRing anyway since the changes were relatively simple.