[sitemap] if last-modified not set in header use current date
[sitemap] if last-modified not set in header use current date

This solves the following issue when last-modified is not set in the
response header:

---snip---
KeyError: 'Last-Modified'
---snap---

Change-Id: I372f9638f6b5006ff250b9dbe0e75cc1556d18c2

openstack/openstack-doc-tools,openstack/openstack-doc-tools