Prefer to pass block when logging.

This follows the good practice listed on http://guides.rubyonrails.org/debugging_rails_applications.html#impact-of-logs-on-performance.
I think using the block form would be less error prone. Right now, this guard and the following call to `debug` are directly coupled. If one changes, the other needs to as well. Using the block version, this guard will be done by the `Logger`. What do you think?