Add support for MySQL INSERT INTO .. SET .. syntax in MERGE statement's WHEN NOT MATCHED THEN INSERT clause

[lukaseder]

This is not supported by any database, but for overall API consistency, for increased typesafety and for more convenience, it might be useful to be able to write

MERGE INTO ..
USING (..)
ON (..)
WHEN MATCHED THEN UPDATE SET
  f1 = v1
  f2 = v2
WHEN NOT MATCHED THEN INSERT SET
  f1 = v1
  f2 = v2