BEMHTML: use proper boolean values for attribute

[tadatuta]

Since bem/bem-xjst#183 boolean attributes are rendered as really boolean:

{ attrs: { 'aria-hidden': true } }

is now rendered without ="true" part which invalidates quite a lot of tmpl specs and as far as I understand some a11y cases, e.g.: https://github.com/bem/bem-components/blob/v2/common.blocks/popup/popup.bemhtml#L12

For aria-* cases it should be like 'aria-hidden': 'true' (fixes in templates) and for other cases (i.e. disabled: 'disabled') should be like disabled: true (fixes in templates AND specs).