Eng 6879 json set field

Creating a pull request for the trivial work of re-enabling the SET_FIELD function, and simplifying tests using the testProcWithValidJSON method in place of the (almost) same 4 lines of code, repeated over and over, since this should be easy to review; the substantial changes for this ticket will come in related branch ENG-6879-more-json-set-field (already in progress).