Handle mkt-user being true/false/falsey (bug 1107700)

This is the Spartacus side to https://github.com/mozilla/webpay/pull/579 see the webpay patch for a description.
I think it would be easier to accept native booleans as input here and convert them to strings. So something like:

     document.body.setAttribute('data-mkt-user', (mktUser === false) ? 'false': (mktUser ? 'true': ''));