[bunyan] Use message field as message

[curusarn]

Logging without message as a separate argument was completely ignored before - e.g. this would produce no log in Live tail:

logger.info({ msg: "Message", moreContext: "more info" })

This PR:

• allows logs without message argument
• uses 'message' field if message argument is missing

Known issue:
Bunyan completely ignores 'msg' field passed inside the context object. I.e. you are not supposed to log like this: logger.({ msg: "Message" })
Some issues about that here:
trentm/node-bunyan#515 (comment)
trentm/node-bunyan#517

[PetrHeinz]

Thanks 🙌 Nice fix, just 2 tiny comments below