Payment instructions are not returned from FE API

[grossmannmartin]

Describe the bug

Payment instructions set in admin are not returned from FE API with Payments query (or any other query returning payment).

To Reproduce

1. In administration fill in instructions for payment
2. Run graphql query for payments

{
  payments {
    name
    instruction
  }
}

3. See payments are null

Expected behavior

Payments are returned properly

Additional context

It seems like the field in graphql type is named in singular (instruction), while getter in the payment class is in plural (getInstructions()).
Renaming the field in graphql type should help, but it will be a BC break. It's possible to introduce a resolverMap for instruction field, or add new getter to payment (getInstruction())

[vitek-rostislav]

Btw, there is a test that is giving us a false sense of security now - see https://github.com/shopsys/shopsys/blob/master/project-base/tests/FrontendApiBundle/Functional/Payment/PaymentsTest.php#L50 - it asserts the instruction is null while in data fixtures, there is a text that should be actually asserted - https://github.com/shopsys/shopsys/blob/master/project-base/src/DataFixtures/Demo/PaymentDataFixture.php#L71. The current test will fail when the problem described above is fixed 🙂