do not overwrite @timestamp in data stream if it already exists in the record

[dschaaff]

closes #967

If the log record already contains the field @timestamp, then the data stream output plugin should not overwrite the field.

(check all that apply)

• [x] tests added
• [x] tests passing
• [ ] README updated (if needed)
• [ ] README Table of Contents updated (if needed)
• [x] History.md and version in gemspec are untouched
• [x] backward compatible
• [ ] feature works in elasticsearch_dynamic (not required but recommended)

[dschaaff]

@cosmo0920 I'll look into adding a test.

[dschaaff]

@cosmo0920 I've added a test if you don't mind reviewing when you have time. Thanks!

[rjbma]

Just ran into the problem outlined in #967 and noticed this PR fixes it. Any news on when this is getting released?

Thanks!