Add spec of mass assigment on Comment
user
```ruby
require 'spec_helper'

describe Comment do

  describe "mass assignment" do

    context "allowed" do
      [:body, :proposal_id].each do |attr|
        it { should allow_mass_assignment_of(attr) }
      end
    end

    context "not allowed" do
      [:id, :user_id, :created_at, :updated_at].each do |attr|
        it { should_not allow_mass_assignment_of(attr) }
      end
    end
  end
end
```
assistant