[FIX] XAU/demmurage not working when flipping or clicking on an order
[FIX] Currency pairs not updated in drop after flipping/clicking on an order
[FIX] Invalid/non-canonical currency pairs saved in drop menu

Fixs a bug in RT-1876 which will leave invalid currency pairs in the drop menu.
Now I use Currency#to_json() to generate canonical names for currency pairs.
Old non-canonical currency pairs will be canonicalized too.
i.e. "xaU - (-0.5%pa)/cNy" will be canonicalized into "XAU (-0.5%pa)/CNY".
Merge pull request #1388 from orzFly/patch-demmurage-trade

[FIX] Problems about demmurage, flipping and clicking on order