Fix encoding problem in expenses tests

@BenoitMC as we use ruby 1.9 in production, using a non ascii character like French accents require to add "# encoding: UTF-8" at the beginning of the file.

By the way, be sure that the rest of the team is ok with writing test scenarios in french. (Just in case you haven't discussed this yet (I'm not working for AGILiDEE so I don't know))