bug of select-single in cascader-demo

[terryatgithub]

hi @adamstep, base on your great implementation #309 , I did some test and found a little maybe bug as below:

When there is a default value set,
What we expected is show level1 and level2 simultaneously.
The fact is below three ways all have some unexpected issue:

1. set attribute value in both select-single, it will run error when select in level1 list:

error-set-value.mov

2. set attribute selected in both select-single, can not show level2 list when enter for the first time:

error-set-selected.mov

3. set attribute value selected separatedly in both select-single, need double click to show both level:

error-set-value-and-selected.mov

[flochtililoch]

@adamstep can you remember if this issue was fixed?