ignore unknown JSON fields when deserializing to a POJO

An external REST API can be extended and stay backwards compatible,
for example just be adding new fields in the JSON result.

The default mapper is configured so that a client using the
java Play JSON API can still consume the external REST API.
Merge pull request #3670 from yanns/tolerant-java-json-mapper

ignore unknown JSON fields when deserializing to a POJO