问如何对Rails邮件程序生成的html内容进行单元测试？

EN

RSpec.describe DigestMailer do
  describe "#daily_digest" do
    let(:mail) { DigestMailer.daily_digest(user.id) }
    let(:user) { create(:user) }

    it "sends from the correct email" do
      expect(mail.from).to eql ["support@example.com"]
    end

    it "renders the subject" do
      expect(mail.subject).to eql "Your Daily Digest"
    end

    it "renders the receiver email" do
      expect(mail.to).to eql [user.email]
    end

    it "renders the number of new posts" do
      expect(mail.body.raw_source).to match "5 New Posts"
    end
  end
end

within ".posts-section" do
  expect(html_body).to have_content "5 New Posts"
  expect(html_body).to have_link "View More"
  expect(find_link("View More").to link_to posts_url
end