David Heinemayer Hanson added activeresource.rb to the code base so people can fulfill their requirements based on the name of the pearl and not have to remember that despite the gem being called activeresource, they need to require active_resource.rb
The original file is still needed by everyone who has already used it, so only the old one is required for the new file.
Check comment for activeresource.rb
Edit:
Found the original upgrade ticket that led to this change
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