revert code change

#67

Author: altendky

src/desert/_make.py

@@ -213,7 +213,10 @@ def field_for_schema(
     if default is not marshmallow.missing:
         desert_metadata.setdefault("default", default)
         desert_metadata.setdefault("allow_none", True)
-        desert_metadata.setdefault("missing", default)
+        if not desert_metadata.get(
+            "required"
+        ):  # 'missing' must not be set for required fields.
+            desert_metadata.setdefault("missing", default)
 
     field = None