Don't empty _session_cache during create on SessionStore

[Remiz]

Starting Django 1.7 the create() method of db/SessionStore removes this line.

I noticed an issue when using the SessionStore manually with Django 1.9 and django-user-sessions:

ipdb>session = import_module(settings.SESSION_ENGINE).SessionStore(some_session_key)
ipdb>session['foo'] = 'bar'
ipdb>print session.get('foo')
bar
ipdb> session.create()
ipdb> print session.get('foo')
None

I'm not 100% sure how it'll impact Django 1.6 and older but considering the support was dropped in 2014, maybe it won't impact so many people.

[coveralls]

Coverage decreased (-0.03%) to 89.931% when pulling ae5080b on Remiz:patch-1 into 8b7a336 on Bouke:master.

[Bouke]

This is the upstream commit where this line was removed on the master branch. And it was also backported in this commit to version 1.8. Seeing that this change was made to the versions that this package currently supports (1.8 and up), this change can be merged. However as there are quite a few upstream changes in those commits, it is prudent to review those changes and see if/how they should be merged.

[codecov]

Codecov Report

Merging #63 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   93.84%   93.81%   -0.03%     
==========================================
  Files          10       10              
  Lines         276      275       -1     
  Branches       31       31              
==========================================
- Hits          259      258       -1     
  Misses         11       11              
  Partials        6        6

Impacted Files	Coverage Δ
user_sessions/backends/db.py	89.39% <ø> (-0.16%)	⬇️

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