Actually trying to check what's being used in the method instead of
checking for something and use another thing. Thanks @jferris.

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Closes #910

is displayed

would cause the best_content_type to throw an error on trying nil.content_type.

s3 url doc improvements

This should fix the problem on animated file detection.

Fixes #851

This should fix an error when the user has turn on AWS logging while
and upload a file with non-English filename, as `aws-sdk` logger is not
encoding aware.

The problem was that the string returned from another logger methods
is in `US-ASCII`, while the string returned from Paperclip would be in
`UTF-8`. When the logger calls `#join` on those strings, Ruby would
raise an exception.

This solution is not the best, as it involves a monkey-patching, but
should make sure that the upload goes through without any hiccup. For
the long run, I'm going to send a patch to `aws-sdk` and also make
Paperclip require at least `aws-sdk > 1.4.1`