[utils] Let request headers override standard headers

What was I thinking when writing this?

youtube_dl/utils.py

@@ -767,8 +767,7 @@ class YoutubeDLHandler(compat_urllib_request.HTTPHandler):
 
     def http_request(self, req):
         for h, v in std_headers.items():
-            if h in req.headers:
-                del req.headers[h]
+            if h not in req.headers:
                 req.add_header(h, v)
         if 'Youtubedl-no-compression' in req.headers:
             if 'Accept-encoding' in req.headers: