Fixed bug in new "ON CONFLICT DO UPDATE" feature where the "set"
values for the UPDATE clause would not be subject to type-level
processing, as normally takes effect to handle both user-defined
type level conversions as well as dialect-required conversions, such
as those required for JSON datatypes. Additionally, clarified that
the keys in the set_ dictionary should match the "key" of the column,
if distinct from the column name. A warning is emitted
for remaining column names that don't match column keys; for
compatibility reasons, these are emitted as they were previously.
Fixes: #3888
Change-Id: I67a04c67aa5f65e6d29f27bf3ef2f8257088d073