Ensure all strings with spaces are quoted in logs

[bensymonds]

The current logic covers the case when the log data is already a String instance, but not when the data is some other object that, when interpolated into the log string, results in a string containing spaces.

The example that led me here is where an exception is logged and the exception's message contains spaces. The exception gets interpolated into the string and has to_s called on it. Exception#to_s returns the message, so the message (with its spaces) go into the log line, but it's bypassed the logic that adds quotes.

See the commits for more details. I slipped in a few cleanups and added some tests too.

[bensymonds]

@stevenharman Thanks for the review. FYI I amended the last commit to add a changelog entry. I'm going to go ahead and merge this. I actually have another PR I want to open, but after that I'll be bugging you for a release 😄