[core-client-rest] Determine how to handle unexpected body shapes

[timovv]

Follow up to this PR thread. The default behavior seems to be to JSON.stringify if we run into a body shape we don't know how to handle: 1, 2, 3. In some cases this might make sense, but in other scenarios e.g. multipart/form-data it does not. Is there a better way to handle this scenario?