Simplified serializing HTTP response headers.

Since ResponseHeaders was introduced, header names and values are stored
as strings. There is no need to check whether they are bytes.

Co-authored-by: Nick Pope <nick@nickpope.me.uk>

django/http/response.py

@@ -153,14 +153,10 @@ class HttpResponseBase:
 
     def serialize_headers(self):
         """HTTP headers as a bytestring."""
-        def to_bytes(val, encoding):
-            return val if isinstance(val, bytes) else val.encode(encoding)
-
-        headers = [
-            (to_bytes(key, 'ascii') + b': ' + to_bytes(value, 'latin-1'))
+        return b'\r\n'.join([
+            key.encode('ascii') + b': ' + value.encode('latin-1')
             for key, value in self.headers.items()
-        ]
-        return b'\r\n'.join(headers)
+        ])
 
     __bytes__ = serialize_headers