Issue #2226267 by fago, xjm, Berdir: Improve default value handling of fields to be consistent.

core/lib/Drupal/Core/Field/FieldDefinition.php

core/lib/Drupal/Core/Field/FieldDefinitionInterface.php

core/lib/Drupal/Core/Field/FieldItemList.php

core/lib/Drupal/Core/Field/FieldItemListInterface.php

...b/Drupal/Core/Field/Plugin/Field/FieldType/StringItem.php

core/modules/aggregator/src/Entity/Feed.php

core/modules/comment/src/Entity/Comment.php

...time/src/Plugin/Field/FieldType/DateTimeFieldItemList.php

...ld/FieldType/ConfigurableEntityReferenceFieldItemList.php

core/modules/field/src/Entity/FieldInstanceConfig.php

core/modules/field/src/Tests/FieldAttachStorageTest.php

...dules/field/tests/modules/field_test/field_test.field.inc

core/modules/node/src/Entity/Node.php

...dules/serialization/src/Tests/EntitySerializationTest.php

core/modules/shortcut/src/Entity/Shortcut.php

...s/system/src/Tests/Entity/EntityFieldDefaultValueTest.php

...dules/system/tests/modules/entity_test/entity_test.module

...modules/entity_test/src/Entity/EntityTestDefaultValue.php

core/modules/taxonomy/src/Entity/Term.php

...in/Field/FieldType/TaxonomyTermReferenceFieldItemList.php