fix json_format for python2.6:

[anandolee]

1, objcect_pair_hook is not supported in python2.6, so duplicated key check is removed in 2.6
2, total_seconds is not suppoted in python2.6, changed to compute seconds directly

[haberman]

I wonder if it would be better to just drop Python 2.6 support. See: http://www.curiousefficiency.org/posts/2015/04/stop-supporting-python26.html

If we could support Python 2.6 without having to compromise the code-base I'd be ok with that. But I'm not sure I want to see Python 2.6-specific hacks in the code-base.

[googlebot]

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[anandolee]

I am open to keep or drop Python2.6

There were 3 diffs:
1, no total_seconds()
2, json.dump give {"True":xxx} instead of {"true":xxx} for bool map
3, no object_pairs_hook support

1 and 2 can be settled easily. Without object_pairs_hook I haven't thought a good way for duplicate keys, python2.6-specific hacks were added.

[googlebot]

CLAs look good, thanks!

[anandolee]

friendly ping @pherl

[liujisi]

LG