DATAMONGO-1012 - Improved identifier initialization on DBRef proxies.

Identifier initalization is now only triggered if field access is used. Before that the id initialization would've resolved the proxy eagerly as the getter access performed by the BeanWrapper would've been intercepted by the proxy and is indistinguishable from a normal method call. This would've rendered the entire use case to create proxies ad absurdum.

Added test case to check for non-initialization in the property access scenario.