JSON::MixedEncoder: turned off ScalarRef exceptions (new warning message)

See in-code comments.

Added new support for handling special ScalarRef values (like \'null')
and now globally translate other, unknown values to undef with a warning
message. Once I have a better handle on when these odd values will be
encountered, this warning may be removed (or will condifently never
happen or be turned back into an exception-level event).