INT-3323: Add poller element to HTTP-outbound

JIRA: https://jira.spring.io/browse/INT-3323

Poller is internally supported in the bean definition of an http outbound gateway and outbound adapter, but it is not supported by their schema. This change attempts to fix the schema.

* Added in schema element for base poller, in both outbound adapter and outbound gateway
* Added in tests to ensure that a PollingConsumer is the instance type
* Polishing `import` for test classes