When setting headers, fallback to utf-8 if no encoding is specified…

… yet. (#820)

* Headers: fallback to utf8 encoding

* Headers.__setitem__: use utf8 only if encoding is not set

httpx/_models.py

@@ -515,8 +515,8 @@ def __setitem__(self, key: str, value: str) -> None:
         Set the header `key` to `value`, removing any duplicate entries.
         Retains insertion order.
         """
-        set_key = key.lower().encode(self.encoding)
-        set_value = value.encode(self.encoding)
+        set_key = key.lower().encode(self._encoding or "utf-8")
+        set_value = value.encode(self._encoding or "utf-8")
 
         found_indexes = []
         for idx, (item_key, _) in enumerate(self._list):