Fix HEAD not being retried for HTTP 500 category responses. See #579

Author: nh2

lib/upload.js

@@ -616,6 +616,14 @@ class BaseUpload {
           this._emitHttpError(req, res, 'tus: upload is currently locked; retry later')
           return
         }
+        if (inStatusCategory(status, 500)) {
+          // Run retry logic if the server has an error, e.g. 502 Bad Gateway when
+          // proxied server is temporarily down. See issue #579.
+          // Since we want to retry, do not clear `this.url` and do not
+          // create a new upload.
+          this._emitHttpError(req, res, 'tus: resuming upload resulted in HTTP 500 category error')
+          return
+        }
 
         if (inStatusCategory(status, 400)) {
           // Remove stored fingerprint and corresponding endpoint,