Handle single quotes in PostgreSQL default column values

PostgreSQL escapes single quotes by using an additional single quote.
When Rails queries the column information, PostgreSQL returns the
default values with the escaped single quotes.

#extract_value_from_default now converts these to one single quote each.

Fixes #10881.
Merge pull request #10884 from dmarkow/pg_fix_default_quotes

Handle single quotes in PostgreSQL default column values