UP-4223:  Enhance ChainingProfileMapperImpl to provide a stickySelection...

... feature

https://issues.jasig.org/browse/UP-4223

Add support for a stickySelection feature: if enabled (it's off by default), a user's choice of a specific profile will be remembered until s/he makes another specific choice. In other words, choosing a non-default profile makes it your own, personal default from there forward.
This doesn't sound right.  Looks to me like this returns the actual `userAttributes` and not a copy, such that if the caller mutates it those changes take effect on this `PersonImpl`.  Should it be returning a copy instead of the member directly?