Fix case of `http_headers`

Bug in 8b7539d27c

Fixes https://github.com/yt-dlp/yt-dlp/issues/1346#issuecomment-1064527765

yt_dlp/utils.py

@@ -5489,4 +5489,4 @@ has_websockets = bool(compat_websockets)
 
 def merge_headers(*dicts):
     """Merge dicts of http headers case insensitively, prioritizing the latter ones"""
-    return {k.capitalize(): v for k, v in itertools.chain.from_iterable(map(dict.items, dicts))}
+    return {k.title(): v for k, v in itertools.chain.from_iterable(map(dict.items, dicts))}