pgsql: Revert removal of trigger flag from plperl function hash key.

Revert removal of trigger flag from plperl function hash key.

As noted by Jan Urbanski, this flag is in fact needed to ensure that the
function's input/result conversion functions are set up as expected.

Add a regression test to discourage anyone from making same mistake
in future.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=76b12e0af765b2ca2eeddcabdd5e7c22eda164da

Modified Files
--------------
src/pl/plperl/expected/plperl_trigger.out | 6 ++++++
src/pl/plperl/plperl.c | 8 +++++++-
src/pl/plperl/sql/plperl_trigger.sql | 7 ++++++-
3 files changed, 19 insertions(+), 2 deletions(-)