Truncate millis larger than 3 digits

[brenuart]

See #1745

[brenuart]

Should we make the truncate behavior optional/configurable ?

[cowtowncoder]

Oh shoot. Somehow I missed this PR -- my apologies. I fixed it along the same lines, of course, but would love to get tests merged (I added tests but much smaller).

As to behavior: I'd be fine leaving it as is for now. As with other aspects (colon or no colon for time zone offset), adding method(s) for creating new instances with different configuration would be fine. But I am not sure I see clear need yet.

[brenuart]

I merged master back into this branch. The only changes left now are those related to the tests. You can safely merge the PR and you'll get the updated test case.