The "Accept header" documentation http://infinispan.org/docs/stable/user_guide/user_guide.html#rest.accept has the following information

will cause Infinispan to try first to return content in JSON format (higher priority 0.8). If it’s not possible to convert the storage format to JSON, next format tried will be text/plain (second highest priority 0.7), and finally it falls back to /, that will pick a format suitable for displaying automatically based on the cache configuration.

But it was not designed to be working on this way.

The commit https://github.com/diegolovison/infinispan/commit/6a57964f26d6742e8ec29cc00ff95c1e272689c5 has an invalid xml content with a second priority format equals "text/plain".
The xml is invalid but the text is valid.