python - be: Add common mapping object methods 'keys' and 'has_key'.

Log Message:
-----------
Add common mapping object methods 'keys' and 'has_key'.

Modified Files:
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be/src:
tupledesc.c (r1.2 -> r1.3)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/tupledesc.c.diff?r1=1.2&r2=1.3)
be/include/pypg:
tupledesc.h (r1.2 -> r1.3)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/include/pypg/tupledesc.h.diff?r1=1.2&r2=1.3)