Fix a bug in instance:update() function in model.lua.
Fix a bug in instance:update() function in model.lua.

Now, after updating new value to db, it also update new value to lua object itself.

Signed-off-by: Daogang Tang <6435653d2db08e7863743a16df53ecbb301fb4b0@gmail.com>

daogangtang/bamboo,daogangtang/bamboo