Add option to detect content encoding to fix gzip uploads. Resolves #451

[skruger]

The behavior selected in the resolution of #263 has broken django storages when trying to upload normal gzip files without double compressing them or when AWS_IS_GZIPPED=False.

AWS_DETECT_CONTENT_ENCODING has been added to allow the behavior that was introduced last year to be used when desired. If AWS_DETECT_CONTENT_ENCODING=True and AWS_IS_GZIPPED=False gzip encoded files will be stored in their uncompressed form on S3. The detect content encoding default in the storage classes is False which reverses the behavior introduced in pull request #264. This default of False would appear to be the least confusing to someone in a new project trying to use django-storages to upload a .gz file.

This pull request modifies the existing unit test and adds one new unit test to verify the behavior of both AWS_DETECT_CONTENT_ENCODING states in both the boto and boto3 based storage classes. The new configuration option has also been added to the documentation.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@87c42d1). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #453   +/-   ##
=========================================
  Coverage          ?   76.08%           
=========================================
  Files             ?       11           
  Lines             ?     1568           
  Branches          ?        0           
=========================================
  Hits              ?     1193           
  Misses            ?      375           
  Partials          ?        0

Impacted Files	Coverage Δ
storages/backends/s3boto.py	87.45% <100%> (ø)
storages/backends/s3boto3.py	86.58% <100%> (ø)

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[skruger]

The Django 1.8 on Python 3.3 unit test appears to be failing because it installed paramiko 2.4.0 which is officially incompatible with Python 3.3. According to the categories metadata on pypi.python.org paramiko 2.3.1 does support python 3.3 so forcing the lower version for python 3.3 installations might work to resolve this unit test failure.

[skruger]

All tests are passing except for the error resolved by pull request #415

[sww314]

@skruger can you rebase this fix?

[skruger]

@sww314 rebase complete. There were some rough conflicts.

[skruger]

@sww314 I saw that there were some new merge conflicts so I rebased again. Still ready for merge or further review.