Determine charset from contentType if any

[bjmi]

There is no guarantee that the consumer uses Jackson to decode the content.

We can consider moving the charset to the contentType; please open a new issue for that.

Originally posted by @garyrussell in #1251 (comment)

Affected version: spring-amqp 2.4.2
Recently we were affected by this as the charset was provided in contentType header like e.g.
content-type: application/xml; charset=utf-16 and causes deserialization problems.
For the sending part: non spring-amqp consumers don't expect to find charset at contentEncoding header.

Please consider moving the charset to the contentType.