Add test and comment.

Signed-off-by: Jakub Martin <kubam@spacelift.io>

transport.go

@@ -64,6 +64,7 @@ func (e *HTTPError) Error() string {
 	return e.Message
 }
 
+// Unwrap implements the standard library's error wrapping. It unwraps to the root cause.
 func (e *HTTPError) Unwrap() error {
 	return e.RootCause
 }

transport_test.go

@@ -3,6 +3,7 @@ package ghinstallation
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"net/http"
@@ -453,3 +454,15 @@ func TestExpiryAccessor(t *testing.T) {
 		})
 	}
 }
+
+func TestHTTPErrorUnwrap(t *testing.T) {
+	wrappedError := errors.New("wrapped error")
+
+	err := &HTTPError{
+		RootCause: wrappedError,
+	}
+
+	if !errors.Is(err, wrappedError) {
+		t.Errorf("HTTPError should be unwrapped to the root cause")
+	}
+}