Update to threadlocals.py middleware to add compatibility with v.1.10+ of Django (fix of earlier pull request #28)

[ruggedness-mime]

This contains the previous updates to threadlocals.py that were contained in my previous pull #28 (which was closed due to a git error on my part) in reference to issue #27.

These changes make the middleware in audiofield compatible with Django 1.10.

The most recent update should work for making threadlocals.py compatible for versions before 1.10 and after 1.10.

Will be testing on pre-1.10 Django versions this week.

[ruggedness-mime]

This fix is a step towards creating the future compatible threadlocals middleware class but will not load yet.

Have been able to get the doc supported fix to work appropriately now - have created the new branch of this clone as 'mixin-fix' that implements that but it is not a long term solution.

[khasanovbi]

Looks like this PR already included, can we close this and related issue?