Add the specs for the helper
user
```ruby
require "spec_helper"

describe S3Sign::Helper do
  include S3Sign::Helper

  describe "#s3_signed_url_for_key" do
    it "calls S3Sign.url with the given key and expires in" do
      expect(S3Sign).to receive(:url).with("test.txt", 86_400).and_return(:url_one)
      expect(s3_signed_url_for_key("test.txt")).to eq(:url_one)

      expect(S3Sign).to receive(:url).with("test.pdf", 3600).and_return(:url_two)
      expect(s3_signed_url_for_key("test.pdf", 3600)).to eq(:url_two)
    end
  end

  describe "#stable_s3_signed_url" do
    it "signs with a default far future expires time of 2036-1-1 00:00:00 UTC" do
      expect(S3Sign).to receive(:url).with("test.txt", Time.parse("2036-01-01 00:00:00 UTC")).and_return(:url)
      expect(stable_s3_signed_url("test.txt")).to eq(:url)
    end

    it "signs for a given time in 2036 if given a reference time" do
      expect(S3Sign).to receive(:url).with("test.txt", Time.parse("2036-03-24 21:47:30 UTC")).and_return(:url)
      expect(stable_s3_signed_url("test.txt", Time.parse("2015-03-24 21:47:30 UTC"))).to eq(:url)
    end
  end
end
```
assistant