Check if the header has been assigned to the request in Go unit-testing

I am trying to test the following line of code:

httpReq.Header.Set("Content-Type", "application/json")

I am mocking the request to an external api in this way:

httpmock.RegisterResponder(http.MethodPost, "do-not-exist.com",
		httpmock.NewStringResponder(http.StatusOK, `{
			"data":{"Random": "Stuff"}}`),
	)

And want to test if the request to the api has the header that I assigned. Is there a way I could achieve this?

With the help of the comment by @Kelsnare I was able to solve this issue in the following way:

	httpmock.RegisterResponder(http.MethodPost, "do-not-exist.com",
			func(req *http.Request) (*http.Response, error) {
				require.Equal(t, req.Header.Get("Content-Type"), "application/json")
				resp, _ := httpmock.NewStringResponder(http.StatusOK, `{
            "data":{"Random": "Stuff"}}`)(req)
				return resp, nil},
		)

I wrote my own func of http.Responder type and used httpmock.NewStringResponder inside that func.

response_test.go illustrates how the header is tested:

		response, err := NewJsonResponse(200, test.body)
		if err != nil {
			t.Errorf("#%d NewJsonResponse failed: %s", i, err)
			continue
		}

		if response.StatusCode != 200 {
			t.Errorf("#%d response status mismatch: %d ≠ 200", i, response.StatusCode)
			continue
		}

		if response.Header.Get("Content-Type") != "application/json" {
			t.Errorf("#%d response Content-Type mismatch: %s ≠ application/json",
				i, response.Header.Get("Content-Type"))
			continue

You can see an example of table-driven test with httpmock.RegisterResponder here.