Nested mixes work wrong with strings

BEMJSON:
```js
{
block: 'b1',
mix: 'b2'
}
```

BEMHTML:
```js
block('b2')(
addMods()('b3')
);
```

Result:
```html

Expected:
```html

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

Start by reproducing the BEMJSON and BEMHTML examples using the linked bem-xjst playground and compare the actual class output with the expected output. Trace how a string mix is processed together with nested addMods() behavior; done means the example produces b1 b2 b3 without the extra classes.

Written by the indexing model from the issue text.