Refs #33546 -- Optimized HttpResponseBase.charset a bit.

This avoids scanning the Content-Type if it's empty, allowing the
Content-Type header itself to have a charset assigned without using
the re module.

django/http/response.py

@@ -152,11 +152,15 @@ class HttpResponseBase:
     def charset(self):
         if self._charset is not None:
             return self._charset
-        content_type = self.get("Content-Type", "")
-        matched = _charset_from_content_type_re.search(content_type)
-        if matched:
-            # Extract the charset and strip its double quotes
-            return matched["charset"].replace('"', "")
+        # The Content-Type header may not yet be set, because the charset is
+        # being inserted *into* it.
+        if content_type := self.headers.get("Content-Type"):
+            if matched := _charset_from_content_type_re.search(content_type):
+                # Extract the charset and strip its double quotes.
+                # Note that having parsed it from the Content-Type, we don't
+                # store it back into the _charset for later intentionally, to
+                # allow for the Content-Type to be switched again later.
+                return matched["charset"].replace('"', "")
         return settings.DEFAULT_CHARSET
 
     @charset.setter