fix: add string fallback for S3#Expires field when not a date-time (#4577)

Author: kuhe

.changes/next-release/bugfix-S3-e7312ed3.json

@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "S3",
+  "description": "add compatibility fallback S3#ExpiresString when S3#Expires field is not a date-time"
+}