Appending headers to Rspec controller tests

I was able to fix it by using @request.env instead of @request.headers like so:

describe ApplyController do
  context 'when valid' do
    let(:parameters) do
      file = File.join File.dirname(__FILE__), '..', 'samples', 'Indeed.json'
      JSON.parse(File.read file)
    end
    let(:signature) { 'GC02UVj0d4bqa5peNFHdPQAZ2BI=' }

    it 'returns 200 ok if Request is valid' do
      @request.env['X-Indeed-Signature'] = signature
      post :indeed, parameters
      expect(response.status).to eq 200
    end
  end
end

I think you want (straight from one of your links)

it "returns 200 ok"
  @request.headers['X-Indeed-Signature'] = signature
  post :indeed, parameters
  response.status.should == 200
end

You don't need subject(:response)