Update HttpClientBuilder.cs (#442)

if get the httpclient from cache,when the save method called ,it can not to save the right httpclient;

src/Ocelot/Requester/HttpClientBuilder.cs

@@ -40,6 +40,7 @@ namespace Ocelot.Requester
 
             if (httpClient != null)
             {
+                _client = httpClient;
                 return httpClient;
             }