Re-fix of #838, #766, #911

IMPORTANT: There is a bug in the current release of Fog (1.3.1) that will
return a broken expiring URL when the bucket name is valid for a subdomain.
This is fixed per https://github.com/fog/fog/pull/857 but not released
in the gem yet. I have added currently FAILING tests for these cases that
should pass when a new Fog is released.

This relates to #817

`#reverse_merge` did not work as planned because :default key was
already existed in the hash.

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.