Comply with content types for the accept header (#1424)

* Verify accept header for payloads with `@mediaType`

Comply with content-types as described in the documentation [0]

[0] https://awslabs.github.io/smithy/1.0/spec/aws/aws-restjson1-protocol.html#content-type



Signed-off-by: Daniele Ahmed <ahmeddan@amazon.com>