PostgreSQL provider won't work on PostgresSQL >= 12.0

[jng]

The PostgreSQL provider makes reference to the column pg_attrdef.adsrc ​here.

This column has been removed in PostgreSQL 12.0, rendering the provider un-usable for this version of PostgreSQL.

The use of d.adsrc can be replaced with pg_get_expr(d.adbin, d.adrelid). For even more safety, do a PG version check to see whether we can use adsrc or its replacement.

[jng]

In 7962:

Fix PostgreSQL 12 compatibility.

Fixes #973

[jng]

In 7964:

Merged revision(s) 7962-7963 from branches/4.1:

Fix PostgreSQL 12 compatibility.
Fixes #973

If CheckReaderString? fails in any of the Expression Engine function tests, lets be at least courteous and record the values being tested that failed expectations.
........