Conditionally keep-alive component cache goes wrong when 'include' prop dynamically changes

### Vue.js version
2.1.8 with vue-router 2.1.1

### Reproduction Link
https://jsfiddle.net/shidianxia/ckj7xbqq/

### Steps to reproduce
a -> b -> c -> Back (b) -> Back(a) -> b -> c -> Back(b)

### What is Expected?
b is cached and enter into activated lifecycle
Component "a,b" Cached

### What is actually happening?
b is cached but mounted a new one
Component "a,b,b" Cached