fix(SortPlugin): amend sorting to all fields

[flamerohr]

Fix sorting to all fields in the args.

This is because in framework DataList each call to ->sort() actually resets previous sorts made, and the result is that only the end sort happens.

Issue

• fixes #542

[GuySartorelli]

Thank you for this. This feels like something which ought to be tested. Could you please add a unit test to validate the sort args are being correctly applied?

[flamerohr]

is an existing unit test for this plugin I can start writing the tests on? I'm not familiar enough with the test setup to jump right in

[GuySartorelli]

Looks like IntegrationTest class in tests/Schema/IntegrationTest.php has a testFilterAndSort() method which is probably an appropriate place for this. There are also some pre-existing test schemas with sorts applied: https://github.com/search?q=repo%3Asilverstripe%2Fsilverstripe-graphql+sort+path%3A%2F%5Etests%5C%2F%2F&type=code

[flamerohr]

@GuySartorelli test added, the diff looks a bit weird because I needed to tweak the other tests to accomodate to a third row added to the db

[GuySartorelli]

Looks good, thank you!