fix handling of illegal characters in header values

pymodmilter/__init__.py

@@ -279,8 +279,8 @@ class ModifyMilter(Milter.Base):
     def header(self, field, value):
         try:
             # remove surrogates
-            field = field.encode("ascii", errors="replace")
-            value = value.encode("ascii", errors="replace")
+            field = field.encode("ascii", errors="surrogateescape")
+            value = value.encode("ascii", errors="surrogateescape")
 
             self._fp.feed(field + b": " + value + b"\r\n")
         except Exception as e:

pymodmilter/base.py

@@ -136,8 +136,5 @@ class MilterMessage(MIMEPart):
 
 
 def replace_illegal_chars(string):
-    """Replace illegal characters in header values."""
-    return string.replace(
-        "\x00", "").replace(
-        "\r", "").replace(
-        "\n", "")
+    """Remove illegal characters from header values."""
+    return "".join(string.replace("\x00", "").splitlines())