Support logging data with nil exception

[kcdragon]

Prior to this PR, if a log was written with a message and data but the exception field was explicitly nil, the data would not be included in the log. So in the following example

log.error("This is a message", nil, { key: 'value' })

"This is a message" would appear in the log but { key: 'value' } would not. I was expecting { key: 'value' } to be in the log.

This PR adds support for logging the data field with exception explicitly being set to nil. I know this is already supported by only using 2 parameters but it seems unintuitive that a nil exception would cause data to not be logged.

[tilfin]

I think it would be better to change the specification to remove nil in the arguments and apply them.
https://github.com/tilfin/ougai/pull/121/files#diff-d672d7d72aee3369fa48c1c9f7c23d546c44c3ebbf3301c63607da08287ea582
L119 msg, ex, data = args.reject(&:nil?)