Introduce an option function type

- Reduce the duplicate code by merging the flows and
determining the flow type by looking at the provided options.
- Options as a function type allows us to validate an individual
an option in its scope and makes it easier to compose the
built-in options with the third-party ones.

transport_test.go

@@ -9,12 +9,18 @@ import (
 
 type mockTokenFetcher struct{ token *Token }
 
+func (f *mockTokenFetcher) Fn() func(*Token) (*Token, error) {
+	return func(*Token) (*Token, error) {
+		return f.token, nil
+	}
+}
+
 func (f *mockTokenFetcher) FetchToken(existing *Token) (*Token, error) {
 	return f.token, nil
 }
 
 func TestInitialTokenRead(t *testing.T) {
-	tr := NewTransport(http.DefaultTransport, nil, &Token{AccessToken: "abc"})
+	tr := newTransport(http.DefaultTransport, nil, &Token{AccessToken: "abc"})
 	server := newMockServer(func(w http.ResponseWriter, r *http.Request) {
 		if r.Header.Get("Authorization") != "Bearer abc" {
 			t.Errorf("Transport doesn't set the Authorization header from the initial token")
@@ -31,7 +37,7 @@ func TestTokenFetch(t *testing.T) {
 			AccessToken: "abc",
 		},
 	}
-	tr := NewTransport(http.DefaultTransport, fetcher, nil)
+	tr := newTransport(http.DefaultTransport, fetcher.Fn(), nil)
 	server := newMockServer(func(w http.ResponseWriter, r *http.Request) {
 		if r.Header.Get("Authorization") != "Bearer abc" {
 			t.Errorf("Transport doesn't set the Authorization header from the fetched token")