Apply type processing to untyped preexec default clause

Apply type processing to untyped preexec default clause

Fixed bug where a SQL-oriented Python-side column default could fail to
be executed properly upon INSERT in the "pre-execute" codepath, if the
SQL itself were an untyped expression, such as plain text.  The "pre-
execute" codepath is fairly uncommon however can apply to non-integer
primary key columns with SQL defaults when RETURNING is not used.

Tests exist here to ensure typing is applied to
a typed expression for default, but in the case of
an untyped SQL value, we know the type from the column,
so apply this.

Change-Id: I5d8b391611c137b9f700115a50a2bf5b30abfe94
Fixes: #3923