Add support for update graph in custom mappers

[slemesle]

Update graph style mapper methods introduced in 0.9, do not call custom mapper with destination bean.

We should now add this kind of calls by enabling specific signature:

public DestinationBean asDestinationBean(SourceBean source, DestinationBean destination) {
   return null;
}

[slemesle]

Immutable custom mapper should be used by default if no update graph method exist for the same source / destination type mapping.