Allow @postfilter to manage custom objects

This is not a bug but an improvement suggestion.

I'd like to use @postfilter annotation to filter data in a collection based on security roles. My problem is that the controller returns a HashMap which contains the collection HashMap<String, List<MyObj>>

By using the @postfilter I get the following exception:
java.lang.IllegalArgumentException: Filter target must be a collection or array type, but was {result=[myObj: []]} because the return type is the HashMap containing the list of MyObj for the key result
This is due to the check in the DefaultMethodSecurityExpressionHandler.filter which checks if the filterTarget is a collection or array.

The best would be to have the opportunity to specify in the SPEL expression the referred object containing the collection.

I'm using Spring Security 3.2.9.RELEASE

The relevant entry point named is DefaultMethodSecurityExpressionHandler.filter; start by tracing its target-type check and the method-security filtering tests. Define and test how a HashMap containing a filtered collection should be addressed, with completion shown by coverage for the reported HashMap<String, List> case and unchanged collection or array behavior.