Bring this transform function into sync with the policy established
by commit 3a382983d.
Also, fix it to make sure that what it drills down to is indeed a
hash, and not some other kind of Perl SV. Previously, the test
cases added here provoked crashes.
Because of the crash hazard, back-patch to 9.5 where this module
was introduced.
Discussion: https://postgr.es/m/28336.1528393969@sss.pgh.pa.us
This provides a mechanism for specifying conversions between SQL data
types and procedural languages. As examples, there are transforms
for hstore and ltree for PL/Perl and PL/Python.
reviews by Pavel Stěhule and Andres Freund
