Added test to block normal user from updating wiki, fixes #397

[500swapnil]

Make sure these boxes are checked before your pull request is ready to be reviewed and merged. Thanks!

• 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

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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!

[jywarren]

Looks perfect. Great job!