Revert removal of trigger flag from plperl function hash key.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 76b12e0af765b2ca2eeddcabdd5e7c22eda164da
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2010-10-31T15:42:51Z
Releases: 9.1.0
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.

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