INT-3279-3.0.x: Fix class tangle

JIRA: https://jira.springsource.org/browse/INT-3279
INT-3285: Fix AMQP `JsonHeaders` Regression

JIRA: https://jira.springsource.org/browse/INT-3285

Spring Integration 3.0 introduced the `JsonHeaders` and populates them
from a request AMQP Message to the SI Message. In the case of the `AmqpInboundGateway` and
`JsonMessageConverter`, if the SI flow doesn't do anything with `JsonHeaders`
they are returned to the `AmqpInboundGateway` for reply Message
and may override correct values provided by `JsonMessageConverter` in the `DefaultAmqpHeaderMapper`.

This fix provides the check for headers from the reply Message's `MessageProperties`
and populates `JsonHeaders` if the `MessageProperties` doesn't contains the `__TypeId__` header already.

Note, it is just a fix for regression to restore previous behavior.
There is maybe a reason to revise (Another JIRA) all standard AMQP headers and populate them from SI `MessageHeaders`
in the `DefaultAmqpHeaderMapper`, only if they haven't been populated by `MessageConverter` before.

**Cherry-pick to 3.0.x**

Conflicts:

	spring-integration-amqp/src/test/java/org/springframework/integration/amqp/inbound/InboundEndpointTests.java

Resolved.