[downloader] Fix invocation of HttpieFD

Closes #3154

yt_dlp/utils.py

@@ -5502,3 +5502,11 @@ has_websockets = bool(compat_websockets)
 def merge_headers(*dicts):
     """Merge dicts of http headers case insensitively, prioritizing the latter ones"""
     return {k.title(): v for k, v in itertools.chain.from_iterable(map(dict.items, dicts))}
+
+
+class classproperty:
+    def __init__(self, f):
+        self.f = f
+
+    def __get__(self, _, cls):
+        return self.f(cls)