Nested mixes as BEMJSON are not processed

```js
({
block: 'b1',
mix: 'b2'
})
```

```js
block('b2').addMix()({ block: 'b3', mix: 'b4' })
```

http://bem.github.io/bem-xjst/?version=8.9.1&engine=BEMHTML&template=block(%27b2%27).addMix()(%7B%20block%3A%20%27b3%27%2C%20mix%3A%20%27b4%27%20%7D)&bemjson=(%7B%0A%20%20%20%20block%3A%20%27b1%27%2C%0A%20%20%20%20mix%3A%20%27b2%27%0A%7D)

Start with the linked BEMHTML 8.9.1 reproduction and run the two provided snippets, focusing on block('b2').addMix() and the nested mix values. Trace how the outer b1/b2 mix and inner b3/b4 mix are processed; done means nested mixes are processed correctly in the generated output. No source file or test is named.

Written by the indexing model from the issue text.