Ensure redent is idempotent (doesn't add 8 spaces to already dedented values).

Author: jaraco

importlib_metadata/_adapters.py

@@ -57,9 +57,10 @@ def __getitem__(self, item):
     def _repair_headers(self):
         def redent(value):
             "Correct for RFC822 indentation"
-            if not value or '\n' not in value:
+            indent = ' ' * 8
+            if not value or '\n' + indent not in value:
                 return value
-            return textwrap.dedent(' ' * 8 + value)
+            return textwrap.dedent(indent + value)
 
         headers = [(key, redent(value)) for key, value in vars(self)['_headers']]
         if self._payload: