Use fixed-width ISO8601 formatting

[htrendev]

This is a minimal change of how the milliseconds part in iso8601 strings is formatted. Currently any trailing zeroes are removed, which produces different length time strings. Depending on tab size settings in terminal, this may results in level and message columns getting out of alignment (not observed in the example below, but happens in terminal if tab size is set to 4).

2017-03-09T17:47:10.499+0100	INFO	msg1
2017-03-09T17:47:10.5+0100	INFO	msg2
2017-03-09T17:47:10.501+0100	INFO	msg3
2017-03-09T17:47:10.51+0100	INFO	msg4

Keeping the trailing zeroes, preserves the alignment of log messages and makes it a bit easier on the eyes.

2017-03-09T17:47:10.499+0100	INFO	msg1
2017-03-09T17:47:10.500+0100	INFO	msg2
2017-03-09T17:47:10.501+0100	INFO	msg3
2017-03-09T17:47:10.510+0100	INFO	msg4

I'm not sure if such a change needs a test, but I'll write one if indeed required. Updated existing test to check for trailing zeroes.

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Thanks for your contribution!

[mention-bot]

@htrendev, thanks for your PR! By analyzing the history of the files in this pull request, we identified @akshayjshah, @peter-edge and @jcorbin to be potential reviewers.

[CLAassistant]

All committers have signed the CLA.

[akshayjshah]

Looks good - thanks for the fix!