More tests for alex innovation json

Added tests of the JSON function SET_FIELD, plus a few more for FIELD, including with dotted path and/or array index notation, and with dot and bracket characters not used in that way (and therefore escaped with a backslash), and using numeric, floating point data.
The more I think about it, the better I feel about the current implementation's handling of the SQL null value argument over the alternatives. We just need to validate and clearly document its effects.