Keep Headers._encoding when constructing from another Headers instance

[FatimahAlhumaidhi]

I’m trying to build a custom client that defaults header encoding to UTF-8 (related: #3238).

While looking at httpx.Headers.__init__, it seems that when you pass an existing Headers instance, it copies ._list but ignores ._encoding.

httpx/httpx/_models.py

Lines 151 to 164 in ae1b9f6

	if isinstance(headers, Headers):
	self._list = list(headers._list)
	elif isinstance(headers, Mapping):
	for k, v in headers.items():
	bytes_key = _normalize_header_key(k, encoding)
	bytes_value = _normalize_header_value(v, encoding)
	self._list.append((bytes_key, bytes_key.lower(), bytes_value))
	elif headers is not None:
	for k, v in headers:
	bytes_key = _normalize_header_key(k, encoding)
	bytes_value = _normalize_header_value(v, encoding)
	self._list.append((bytes_key, bytes_key.lower(), bytes_value))
	self._encoding = encoding

If I do Headers(existing_headers) (what BaseClient._merge_headers does), I expected it to behave like a copy — including keeping the same encoding — unless I explicitly pass encoding=.

I believe it should carry over its encoding (unless encoding is provided):

        self._encoding = None

        if isinstance(headers, Headers):
            self._list = list(headers._list)
            self._encoding = encoding if encoding is not None else headers._encoding
        ...
        self._encoding = encoding if encoding is not None else self._encoding

If this is intentional, what’s the recommended way to make a client default to UTF-8 header encoding without losing it when headers get copied around?