Save detected non-Latin1 characters as iTXt to preserve them.

PIL/PngImagePlugin.py

@@ -191,12 +191,15 @@ class PngInfo:
             return self.add_itxt(key, value, value.lang, value.tkey, bool(zip))
 
         # The tEXt chunk stores latin-1 text
+        if not isinstance(value, bytes):
+            try:
+                value = value.encode('latin-1', 'strict')
+            except UnicodeError:
+                return self.add_itxt(key, value, zip=bool(zip))
+
         if not isinstance(key, bytes):
             key = key.encode('latin-1', 'strict')
 
-        if not isinstance(value, bytes):
-            value = value.encode('latin-1', 'replace')
-
         if zip:
             import zlib
             self.add(b"zTXt", key + b"\0\0" + zlib.compress(value))