Fix S3 safe_join() to allow colons

[jdufresne]

Combine the identical s3boto3 and s3boto implementations of safe_join()
and its tests to reduce code duplication.

Fixes #248

[codecov-io]

Codecov Report

Merging #322 into master will decrease coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
- Coverage    75.7%   75.51%   -0.19%     
==========================================
  Files          11       11              
  Lines        1552     1540      -12     
==========================================
- Hits         1175     1163      -12     
  Misses        377      377

Impacted Files	Coverage Δ
storages/backends/s3boto3.py	84.63% <100%> (-0.5%)	⬇️
storages/utils.py	100% <100%> (ø)	⬆️

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[jleclanche]

Why is this using urljoin at all instead of os.path.join? Clearly the input isn't URL path components, otherwise behaviour would be broken (as it is).

[jdufresne]

Good question. I think using posixpath.join() does make more sense and actually simplifies the implementation. I have updated the PR with this change.

[jleclanche]

Looks good. Can you think of any other scenarios not covered with the current tests? Changing this method feels a little stressful all in all.

[jdufresne]

Can't think of any at the moment. I added a new test for utils.safe_join("base", "/etc/passwd") as well as fix a typo in test test_trailing_slash_multi(). If you can think of another test to write, I'll happily add it. Let me know.

[jleclanche]

Happy with this.