Allow non-array mixes

BEMJSON: `{ block: 'foo', mix: { block: 'bar' } }`
BEMHTML: `block foo, mix: { block: 'foobar' }`
Result: `TypeError: Object #<Object> has no method 'concat'`
Expected: `<div class="foo bar foobar"></div>`

Correct solution:
https://github.com/bem/bem-core/blob/a5efc85cb/common.blocks/i-bem/i-bem.bemhtml#L360