Fix bug in suppress_emit_error_log_interval

[patrobinson]

Issue

When setting --emit-error-log-interval command line parameter, fluent instead prints the following error which masks other errors:

fluentd_1       | 2018-07-12 23:18:26 +0000 [error]: #0 failed to emit fluentd's log event tag="fluent.error" event={"host"=>"172.20.0.1", "port"=>42300, "error"=>"#<ArgumentError: comparison of Time with 1531437277 failed>", "message"=>"unexpected error on reading data host=\"172.20.0.1\" port=42300 error_class=ArgumentError error=\"comparison of Time with 1531437277 failed\"", "time"=>"2018-07-12T23:18:26Z"} error_class=ArgumentError error="comparison of Time with 1531437277 failed"

Fix

Ensure time is in epoch format before comparison
Add tests

[patrobinson]

I'm not sure why the Windows tests are failing, it doesn't seem to have anything to do with my changes

[repeatedly]

Yes. Test failure is caused by appveyor's unstable execution.

[repeatedly]

Thanks for the patch!