pgsql: Use data-type specific conversion functions also in plpy.execute

Log Message:
-----------
Use data-type specific conversion functions also in plpy.execute

In PLy_spi_execute_plan, use the data-type specific Python-to-PostgreSQL
conversion function instead of passing everything through InputFunctionCall
as a string.  The equivalent fix was already done months ago for function
parameters and return values, but this other gateway between Python and
PostgreSQL was apparently forgotten.  As a result, data types that need
special treatment, such as bytea, would misbehave when used with
plpy.execute.

Modified Files:
--------------
    pgsql/src/pl/plpython:
        plpython.c (r1.139 -> r1.140)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c?r1=1.139&r2=1.140)
    pgsql/src/pl/plpython/expected:
        plpython_types.out (r1.5 -> r1.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_types.out?r1=1.5&r2=1.6)
        plpython_types_3.out (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_types_3.out?r1=1.1&r2=1.2)
    pgsql/src/pl/plpython/sql:
        plpython_types.sql (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/sql/plpython_types.sql?r1=1.4&r2=1.5)