Trouble with millisecond (or any sub-second) resolution (outdated protocol implementation?)

First, thanks for maintaining this module.

Then, it appears to have trouble with getting sub-second resolution right, and it may be because the last update of the related code (#47, 2016) doesn't seem to conform to the current fluentd spec (https://github.com/fluent/fluentd/wiki/Forward-Protocol-Specification-v1, 2017).

I've checked this with a new project having the most recent versions of everything:
- second resolution works fine
- millisecond resolution yields invalid dates
- fluentd is barely configured
- other components (e.g. a ruby one with a "standard" client lib) perform as expected (with nanosecond resolution).

I believe it's a quick fix for @sveneh or @okkez or other people doing great job here.