Don't strip microseconds in JSON output.

[tomchristie]

Closes #4255.

[codecov-io]

Current coverage is 91.23%

Merging #4256 into master will increase coverage by 0.03%

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[kevin-brown]

There was a relevant comment in the original patch about why the microseconds are being truncated. It appears to have been lost throughout the years.

For Date Time string spec, see ECMA 262
http://ecma-international.org/ecma-262/5.1/#sec-15.9.1.15

Which appears to have been copied out of the Django JSON serializer.

In ECMA 262, the datetime string format is defined as YYYY-MM-DDTHH:mm:ss.sssZ, with only 3 decimal precision.

[tomchristie]

The JSON spec has no guidance on datetime formatting. https://tools.ietf.org/html/rfc7159

The current default is for DateTimeField to coerce to strings, so .ssssss is our current behavior unless you explicitly return a datetime, or unless you've set DATETIME_FORMAT to None in the settings.

This change ensures that we're consistent throughout. Users can still set DATETIME_FORMAT if they want a different behavior.

[tomchristie]

But yes, very much wasn't sure on that when you re-raised. And there is the reading should be that JSON should be a subset of ECMA, but that seems pretty loose, stated as a design goal rather than a requirement. (and it's also not quite a subset)