## Context

While working on the `Subscriptions::UpdateService`, I noticed some test cases missing so I decided to add them. Adding the following spec failed:

```ruby
      context "when specifying only name" do
        let(:subscription) { create(:subscription, customer:) }
        let(:params) { {name: "new name"} }

        it "updates the subscription" do
          result = update_service.call

          expect(result).to be_success

          expect(result.subscription.name).to eq("new name")
          expect(result.subscription.subscription_at.to_s).not_to include("2022-07-07")
        end
      end
```

This is due to the fact that `subscription_at` is `nil` in the factory but:

1. It is expected to be set in the `Subscriptions::UpdateService`
2. It is always set when creating a subscription https://github.com/getlago/lago-api/blob/9eebd429260041b822b1bf46abf51a2b200a54b9/app/services/subscriptions/create_service.rb#L13

## Description

This commit adds a default `subscription_at` to the `subscription` factory.