remove old auth header on Auth() call

request.go

@@ -153,7 +153,10 @@ func HeadersMap(headers map[string]string) RequestModifier {
 }
 
 func Auth(auth AuthProvider) RequestModifier {
-	return Headers("Authentication", auth())
+	return chainMods(
+		RemoveHeaders("Authentication"),
+		Headers("Authentication", auth()),
+	)
 }
 
 func Basic(username, password string) AuthProvider {

utils.go

@@ -0,0 +1,10 @@
+package ezhttp
+
+func chainMods(mods ...RequestModifier) RequestModifier {
+	return func(r *RequestBuilder) *RequestBuilder {
+		for _, mod := range mods {
+			r = mod(r)
+		}
+		return r
+	}
+}