Consistently use the same OkHttpClient for files.slack.com calls (#1428)

Author: seratch

slack-api-client/src/main/java/com/slack/api/methods/impl/FilesUploadV2Helper.java

@@ -31,7 +31,7 @@ public class FilesUploadV2Helper implements AutoCloseable {
 
     public FilesUploadV2Helper(MethodsClient client) {
         this.client = client;
-        this.okHttpClient = SlackHttpClient.buildOkHttpClient(client.getSlackHttpClient().getConfig());
+        this.okHttpClient = client.getSlackHttpClient().getOkHttpClient();
     }
 
     public String uploadFile(

slack-api-client/src/main/java/com/slack/api/util/http/SlackHttpClient.java

@@ -123,6 +123,10 @@ public void setConfig(SlackConfig config) {
         this.config = config;
     }
 
+    public OkHttpClient getOkHttpClient() {
+        return this.okHttpClient;
+    }
+
     public Response get(String url, Map<String, String> query, String token) throws IOException {
         if (query != null) {
             HttpUrl.Builder u = HttpUrl.parse(url).newBuilder();