Avoid data race when using http.DefaultClient (#78)

extra_test.go

@@ -115,7 +115,7 @@ type RedirectSuite struct{ suite.Suite }
 func (s *RedirectSuite) TestAddRedirectFunctionality() {
 	client := &http.Client{}
 	ro := &RequestOptions{RedirectLimit: 2, SensitiveHTTPHeaders: map[string]struct{}{"Foo": {}}}
-	addRedirectFunctionality(client, ro)
+	client = addRedirectFunctionality(client, ro)
 	req1 := httptest.NewRequest("GET", "http://x", nil)
 	req1.Header.Set("Foo", "bar")
 	req2 := httptest.NewRequest("GET", "http://y", nil)

request.go

@@ -194,7 +194,7 @@ func buildRequest(httpMethod, url string, ro *RequestOptions, httpClient *http.C
 	addHTTPHeaders(ro, req)
 	addCookies(ro, req)
 
-	addRedirectFunctionality(httpClient, ro)
+	httpClient = addRedirectFunctionality(httpClient, ro)
 
 	if ro.Context != nil {
 		req = req.WithContext(ro.Context)

utils.go

@@ -51,11 +51,13 @@ var (
 // because Go's XML library only supports XML encoded in UTF-8
 type XMLCharDecoder func(charset string, input io.Reader) (io.Reader, error)
 
-func addRedirectFunctionality(client *http.Client, ro *RequestOptions) {
+func addRedirectFunctionality(client *http.Client, ro *RequestOptions) *http.Client {
 	if client.CheckRedirect != nil {
-		return
+		return client
 	}
-	client.CheckRedirect = func(req *http.Request, via []*http.Request) error {
+
+	copied := *client
+	copied.CheckRedirect = func(req *http.Request, via []*http.Request) error {
 
 		if ro.RedirectLimit < 0 {
 			return http.ErrUseLastResponse
@@ -86,6 +88,8 @@ func addRedirectFunctionality(client *http.Client, ro *RequestOptions) {
 
 		return nil
 	}
+
+	return &copied
 }
 
 // EnsureTransporterFinalized will ensure that when the HTTP client is GCed