`#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.

        ,----,      _____________
   ___.`      `,   /             \
   `===  D     :  <  Quack Quack! |
     `'.      .'   \_____________/
        )    (                   ,
       /      \_________________/|
      /                          |
     |                           ;
     |               _____       /
     |      \       ______7    ,'
     |       \    ______7     /
      \       `-,____7      ,'   jgs
^~^~^~^`\                  /~^~^~^~^
  ~^~^~^ `----------------' ~^~^~^
 ~^~^~^~^~^^~^~^~^~^~^~^~^~^~^~^~

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