Cannot edit uesr in ActiveAdmin when username contains '.'

[OmeletWithoutEgg]

As an example, if one visits https://tioj.ck.tp.edu.tw/admin/users/ioi.jqbpzk, he'll get a 406 ERROR. The passenger error log says that it cannot recognize the format (extension).
Also if there's an user whose username ends with .json, it'll send a json with 406 error. e.g. https://tioj.ck.tp.edu.tw/users/abc.json

[adrien1018]

Fixed on #23.