The error message should contain the key as given, not normalized.

Backout of 2e047702df7f.  Reported by Jeremy Kloth.

Lib/packaging/metadata.py

@@ -229,8 +229,10 @@ class Metadata:
 
     def __delitem__(self, name):
         field_name = self._convert_name(name)
-        # we let a KeyError propagate
-        del self._fields[field_name]
+        try:
+            del self._fields[field_name]
+        except KeyError:
+            raise KeyError(name)
         self._set_best_version()
 
     def __contains__(self, name):