Fixed #23384 -- Allowed overriding part of a dictionary-type setting

This change is needed for upcoming changes where settings might be
grouped in a parent dictionary.
@claudep by all means, that's what it's there for. i should throw something BSD-ish on my blog's about page as i get questions about it from time to time. 

depending on the details of use you may want to skim the comments on the post as there's some discussion in there. from what i glean here you seem to be using it for configuration information here so the code is the post is likely to be a good fit as-is.