```ruby
# frozen_string_literal: true

require 'spec_helper'

describe FormatHelper, type: :helper do

  describe 'markdown' do
    it 'should return empty string for nil' do
      expect(markdown(nil)).to eq ''
    end

    it 'should return HTML for header markdown' do
      expect(Redcarpet::Markdown).to receive(:new)
      .with(Redcarpet::Render::HTML, autolink: true, space_after_headers: true, no_intra_emphasis: true, fenced_code_blocks: true, disable_indented_code_blocks: true, safe_links_only: true)
      .and_call_original

      expect(markdown('# this is my header')).to eq "<h1>this is my header</h1>\n"
    end
  end
end
```

Remove test of implementation details
user
```ruby
# frozen_string_literal: true

require 'spec_helper'

describe FormatHelper, type: :helper do

  describe 'markdown' do
    it 'should return empty string for nil' do
      expect(markdown(nil)).to eq ''
    end

    it 'should return HTML for header markdown' do
      expect(markdown('# this is my header')).to eq "<h1>this is my header</h1>\n"
    end
  end
end
```
assistant