Add spec for AccountController.update_credentials

lib/pleroma/web/api_spec/helpers.ex

lib/pleroma/web/api_spec/operations/account_operation.ex

lib/pleroma/web/api_spec/schemas/account_field_attribute.ex

...eb/api_spec/schemas/account_update_credentials_request.ex

...leroma/web/mastodon_api/controllers/account_controller.ex

test/support/conn_case.ex

test/web/api_spec/account_operation_test.exs

...ontrollers/account_controller/update_credentials_test.exs