fix problem with empty media tarball name and boto3 s3 storage

[pacahon]

I've switched to new boto3 s3 storage implementation from django-storages and realized, that it doesn't work due to empty tarball name. This PR should fix it. Maybe it's relevant to dbbackup cmd also.

[ZuluPro]

Hello @pacahon,
Thank you very much for your work.
Maybe the comments are useless, did you test with other storages ?

[coveralls]

Coverage increased (+0.005%) to 96.234% when pulling 70c00db on pacahon:master into 9bab10a on django-dbbackup:master.

[pacahon]

@ZuluPro Now I'm not sure it's the right place to fix unpredictable behavior.
More it looks like hack, because django's File.name attribute is The name of the file including the relative path from MEDIA_ROOT.

Django storage save method accepts any file-like object https://code.djangoproject.com/ticket/27145, so all should be ok from our side. But bug is still here. Maybe it can be fixed in Django later https://code.djangoproject.com/ticket/27150#ticket but should investigate the problem deeply first.

I can try to send patch to django-storages first, but really not sure how fast they can reply.

[ZuluPro]

@pacahon: Try to send a patch, I am also maintener of Django-Storage...

[pacahon]

@ZuluPro Ok, I'll try! :) Saw you in contributors, but didn't realize you can merge PR too.
Mb any thoughts on jschneier/django-storages#194 ?

[ZuluPro]

Hello @pacahon
Is this issue is still relevant ?

[ZuluPro]

@pacahon Any news ?

[pacahon]

Sorry, but I can't remember in what case I had this problem. Since this PR wasn't merged, but I'm still using dbbackup, I definitely did something with this issue (maybe only on my side)