DATAMONGO-1001 - Fix saving lazy loaded object.

With 8f60c16 we now resolve the target type for Cglib proxied objects and initialize lazy loaded ones before saving. As it turns out `CustomConversions` already knows how to deal with proxies correctly, so we've just added some tests for verification.
This commit should be back ported to `1.5.x` though there are some conflicts to resolve.

167648b renames `LazyLoadingProxy.initialize()` to `getTarget()`. Should *NOT* be applied to  `1.5.x`.