Omit "_core" key from normalized config entities, which includes the default config hash

#2795109: [BUGFIX] Support configuration entities with array values explicitly mentions _core. We should look into that issue, investigate whether some of the changes made there need to be re-evaluated.

This is also related to #2860350: Document why JSON API only supports @DataType-level normalizers.

#2915414: Omit "_core" key from normalized config entities, which includes the default config hash landed in core, time to mirror this behavior in JSON API!

Just patch for now, AFAICT JSON API has no test coverage for config entities yet…

Relevant issue for the GraphQL module: https://github.com/drupal-graphql/graphql/issues/64

I meant no functional test coverage. We added functional test coverage for this in core at \Drupal\Tests\rest\Functional\EntityResource\ConfigurableLanguage\ConfigurableLanguageResourceTestBase::testGetDefaultConfig().

We'll also need denormalization test coverage, which core only did via a unit test (\Drupal\Tests\serialization\Unit\Normalizer\ConfigEntityNormalizerTest::testDenormalize()). Only a unit test, because core doesn't yet have full support for config entity denormalization.

It's easy to have unit tests that you think are testing the right thing, but actually aren't. We've all seen that happen I think.

That's why it's important to have functional tests for at least the common cases. It's okay for them to be testing fewer edge cases than unit tests. So I do think missing is the appropriate word.

Well, we might now as well block this on #2930028: Comprehensive JSON API integration test coverage phase 1: for every entity type, individual resources only. Then we'll be able to add explicit integration test coverage that's similar to #2915414: Omit "_core" key from normalized config entities, which includes the default config hash.

#2932031-32: Comprehensive JSON API integration test coverage phase 2: for every entity type, complex use-cases is adding test coverage for this already, but for the wrong behavior in HEAD (based on the test coverage that #2915414: Omit "_core" key from normalized config entities, which includes the default config hash added to core). That means that this issue will simply be able to change one test assertion and we'll have the desired integration test coverage :)

Keeping at "PP-1" despite #2930028 having landed, because this is now blocked on #2932031.

#2932031: Comprehensive JSON API integration test coverage phase 2: for every entity type, complex use-cases landed!

Per #15, test coverage is now trivial to add :)