node.replace unit and functional tests -- TDD!

[jywarren]

Leading with tests!

• all tests pass -- rake test:all
• code is in uniquely-named feature branch, and has been rebased on top of latest master (especially if you've been asked to make additional changes)
• pull request is descriptively named with #number reference back to original issue
• if possible, multiple commits squashed if they're smaller changes
• reviewed/confirmed/tested by another contributor or maintainer
• schema.rb.example has been updated if any database migrations were added

Please be sure you've reviewed our contribution guidelines at https://publiclab.org/wiki/contributing-to-public-lab-software

We have a loose schedule of reviewing and pulling in changes every Tuesday and Friday, and publishing changes on Fridays. Please alert developers on plots-dev@googlegroups.com when your request is ready or if you need assistance.

Thanks!