Make ISO serializers for LocalTime/LocalDateTime more predictable

[dkhalanskyjb]

Now, they will always include seconds and will not add trailing zeros for prettiness to the fractional part.

Fixes #351

[ilya-g]

IMO, this does not solve consistency problem, so we should first discuss it.

[dkhalanskyjb]

Thanks, this was a good call! After an internal discussion, we determined that in general, this wouldn't help a lot to interface with our serializers, given that fractional parts are still optional. For example, in Python, this would lead to code like this anyway: https://stackoverflow.com/questions/30584364/python-strptime-format-with-optional-bits A better way to address this concern seems to be to provide custom serializers and remove the ISO-8601 serializers altogether. We could still provide toString()/parse()-powered ones by some other name.