The `spring.integration.poller.fixed-rate` property must be set to the
constructor of the `PeriodicTrigger` and its `fixedRate` flag should be
set to `true`. The current code-base has it exactly opposite: the flag
is set to `true` when `fixed-delay` is provided.
* Fix `IntegrationAutoConfiguration.asTrigger()` method for the proper
`fixedRate` setting logic.
* Cover the change with a new test-case
* Add a message handling verification to the `defaultPoller()` test to
be sure that poller auto-configuration works as it is claimed.
See gh-28237
769b5039b6