1.7.x
Spring AMQP
Tests: 762 passed
Duration: 4 minutes
Changes: Who: Gary Russell
Why: GH-1236: Handle non-String contentType
Resolves https://github.com/spring-projects/spring-amqp/issues/1236

The user may set the contentType to a `MimeType`.

Always use `toString()` when replacing the `contentType`.

**cherry-pick to 2.2.x, 2.1.x, 1.7.x**

(cherry picked from commit 8c7e4f95ee24fc60ff634d00fe4fc5858f728d88)

# Conflicts:
#        spring-rabbit/src/test/java/org/springframework/amqp/rabbit/annotation/ContentTypeDelegatingMessageConverterIntegrationTests.java
Rebuilt by Artem Bilan