@timestamp doesnt get printed when specfied in message codec

[vjsamuel]

For the following output config:

output.console:
  codec.format:
    string: '%{[@timestamp]} %{[message]}'

the output is always empty because there is no switch case in the formatevents to handle common.Time

This PR fixes that.

[elasticmachine]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run.

[elasticmachine]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run.

[vjsamuel]

@urso can you please provide review comments?

[ruflin]

LGTM. Waiting for @urso to be back from vacation to also have a look.

[urso]

Some notes about @timestamp, not to be solved in this PR:

• MarshalJSON normalizes the @timestamp to UTC, but String() and the EventFormatter does not. That is, time zones are not used consistently in beats.
• unfortunately dtfmt does not support timezone yet, but I'd prefer formatting (in MarshalJSON and String()) done via dtfmt package. Difference is, dtfmt pre-builds the formatter, but stdlib formatting needs to parse the format string and builds the formatter every single time (well, minor detail)

Current PR seems to be most consistent solution as of now.