s3boto modified_time fails for new file 

This isn't actually my fix - I stumbled across it when trying to debug an issue with Mezzanine's usage of Django-Storages and involves a case where last_modified is not set for newly uploaded files.

https://bitbucket.org/david/django-storages/issue/163/s3boto-modified_time-fails-for-new-file

The PR towards original django-storages that I borrowed this fix from...  seems like it's over a year old and never merged.  

https://bitbucket.org/david/django-storages/pull-request/96/if-entrylast_modified-is-none-set-to/diff