resolves#1584
BREAKING CHANGE:
Changed output of RPCs to match return type better:
* single scalar return: value (unchanged)
* setof scalar return: array of values (new; was array of objects)
* single composite return: object (new; was array of objects)
* setof composite return: array of objects (unchanged)
* void return: nothing (new; was "null")
A single OUT column is now treated as "composite" instead of "scalar",
i.e. consistent with multiple OUT columns.
