allow current_user and params to remain lazy in rspec contexts

[zvkemp]

accessing current_user and params in an around block potentially inverts the order of side effects in previously-lazy rspec variable definitions. For example, in tests that change configuration based on example metadata, that configuration is not yet loaded at the during of RSpec's around callback phase. Since those two attributes seem to be a special case, we can just call them as methods on the RSpec context, and otherwise keep the openstruct behavior for other arbitrary attributes.