Add `i-bem` class when mixed element has `js: true` with `elemJsInstances: true`

[tadatuta]

Input code or something about issue background

{
    block: 'b',
    content: {
        elem: 'e1',
        mix: { elem: 'e2', js: true }
    }
}

Expected Behavior

<div class="b"><div class="b__e1 b__e2 i-bem" data-bem='{"b__e2":{}}'></div></div>

Actual Behavior

<div class="b"><div class="b__e1 b__e2" data-bem='{"b__e2":{}}'></div></div>

(no i-bem class).

bem-xjst version and all relevant details
bem-xjst 7.x / 8.x, with elemJsInstances: true