Fix RSpec warning about false positives
Fix RSpec warning about false positives

Using `not_to raise_error` with a specific class can hide problems with
the code causing errors of another kind so follow the recommended
practice of not using a specific class.

joelanman/e-petitions,oskarpearson/e-petitions,alphagov/e-petitions,alphagov/e-petitions,joelanman/e-petitions,StatesOfJersey/e-petitions,unboxed/e-petitions,StatesOfJersey/e-petitions,unboxed/e-petitions,oskarpearson/e-petitions,unboxed/e-petitions,joelanman/e-petitions,StatesOfJersey/e-petitions,StatesOfJersey/e-petitions,oskarpearson/e-petitions,alphagov/e-petitions,alphagov/e-petitions,unboxed/e-petitions