INT-3300: Fix `MS` ref Regression for i-c-adapter

JIRA: https://jira.springsource.org/browse/INT-3300

The change introduced by https://jira.springsource.org/browse/INT-3147
(Registration of `MessageSource<?>` as a bean with suffix `.source`) broke
the case where `MessageSource<?>` is a reference to an existing bean.

Add check to the `AbstractPollingInboundChannelAdapterParser` for the type
of `source` and apply the appropriate logic.

Conflicts:

	spring-integration-core/src/test/java/org/springframework/integration/config/ChannelAdapterParserTests.java

Resolved.