pgsql: Use EncodeDateTime instead of to_char to render JSON timestamps.

Use EncodeDateTime instead of to_char to render JSON timestamps.

Per gripe from Peter Eisentraut and Tom Lane.

The output is slightly different, but still ISO 8601 compliant: to_char
doesn't output the minutes when time zone offset is an integer number of
hours, while EncodeDateTime outputs ":00".

The code is slightly adapted from code in xml.c

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ab14a73a6ca5cc4750f0e00a48bdc25a2293034a

Modified Files
--------------
src/backend/utils/adt/json.c | 77 ++++++++++++++++++++++------------
src/test/regress/expected/json.out | 6 +--
src/test/regress/expected/json_1.out | 6 +--
3 files changed, 56 insertions(+), 33 deletions(-)