Fix GovukContentSecurityPolicy test

[AgaDufrat]

The test started failing because Rails now prevents meaningless assignments to a configuration key by raising NoMethodError. See: rails/rails@265bfad

  GovukContentSecurityPolicy.configure creates a policy
     Failure/Error: Rails.application.config.content_security_policy = nil

     NoMethodError:
       Cannot assign to `content_security_policy`, it is a configuration method

                   raise NoMethodError.new("Cannot assign to `#{key}`, it is a configuration method")
                   ^^^^^
     # ./spec/lib/govuk_content_security_policy_spec.rb:14:in `block (3 levels) in <top (required)>'

[robinjam]

Much better!