pgsql: Fix aboriginal mistake in plpython's set-returning-function supp

Fix aboriginal mistake in plpython's set-returning-function support.

We must stay in the function's SPI context until done calling the iterator
that returns the set result. Otherwise, any attempt to invoke SPI features
in the python code called by the iterator will malfunction. Diagnosis and
patch by Jan Urbanski, per bug report from Jean-Baptiste Quenot.

Back-patch to 8.2; there was no support for SRFs in previous versions of
plpython.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=add0ea88e7722b48d1f3a7c38e3cfd7f1e854674

Modified Files
--------------
src/pl/plpython/expected/plpython_setof.out | 17 ++++++++++++++++
src/pl/plpython/plpython.c | 28 +++++++++++++++++++-------
src/pl/plpython/sql/plpython_setof.sql | 11 ++++++++++
3 files changed, 48 insertions(+), 8 deletions(-)