Fix for find user which start with numeric character

Hello.
Thank you for the service! I am looking forward to future. 

When called `User.find_by_id_or_username ` with start with numeric character, It seems that it searches by user id. 

Example...

access this url: http://www.photographer.io/en/u/03w (my friend)
expect: http://www.photographer.io/en/u/1306
actual: http://www.photographer.io/en/u/3

What do you think?

Sorry about my poor English :)