Support %iso8601 special case for time_format. fix #1528

[repeatedly]

Ruby's iso8601 parser is a restrict version and it doesn't support 20170101T120000+0900 like format.
But this parser can handle 2017-01-01T12:00:00+09:00 and 2017-01-01T12:00:00.123+09:00 cases, so supporting it makes configuration simpler for iso8601 logs.

[repeatedly]

@mururu Could you check this patch?

[mururu]

(EDITED) This case doesn't consider timezone/offset. In other words, this format interprets "2017-01-01T12:00:00" as UTC. Is it correct? (I don't know the detail of ISO8601.)

[repeatedly]

Yes. I assume timestamp has a offset because iso8601 defines offset in the spec.

[repeatedly]

If we need to handle timezone seprately, we can add timezone parameter handling later.

[mururu]

Sorry, I misunderstood Time.iso8601. When the function gets a timestamp without an offset or "Z", it considers the timezone as (fluentd's) localtime. If we want to handle the timestamp as UTC, we have to add Time.now.localtime.utc_offset to the result of Time.iso8601 like the strptime case.

Yes. I assume timestamp has a offset because iso8601 defines offset in the spec.

You mean the spec says the timestamp must have the offset or "Z"? It sounds good.
I think it is better to write the fact the iso8601 parser accept the timestamp without offset and the timezone is recognized as the same with server's localtime in the document.�

[mururu]

anyway, LGTM.

[repeatedly]

Yeah, we should mention the timezone in the document.