Unknown option: storeURL (bug 1045716)

Original problem description is:

With the latest code and with an HTML page that runs this: 

    mozmarket.receipts.Prompter({
        storeURL: "https://marketplace.mozilla.org/app/myapp",
        supportHTML: '<a href="mailto:me@example.com">email me@example.com</a>',
        verify: true
    });

I get this error: Unknown option: storeURL. 

After experimenting I find that:

    mozmarket.receipts.Prompter({
        storeURL: "https://marketplace.mozilla.org/app/myapp",
        supportHTML: '<a href="mailto:me@example.com">email me@example.com</a>',
        verify: false
    });

    "Unknown option: storeURL"
    
    new mozmarket.receipts.Prompter({
        storeURL: "https://marketplace.mozilla.org/app/myapp",
        supportHTML: '<a href="mailto:me@example.com">email me@example.com</a>',
        verify: false
    });

    Prompter {overlay: null, storeURL: "https://marketplace.mozilla.org/app/myapp", supportHTML: "<a href="mailto:me@example.com">email me@example.com</a>", allowNoInstall: false, ignoreInternalError: false…}

There is a line designed to catch this here: https://github.com/mozilla/receiptverifier/blob/master/receiptverifier-ui.js#L88-L90

However I think that the test was malformed, example:

    this instanceof Prompter
    false

    ! this instanceof Prompter
    false

    ! (this instanceof Prompter)
    true

I think this should solve the problem.  Example:

    mozmarket.receipts.Prompter({
        storeURL: "https://marketplace.mozilla.org/app/myapp",
        supportHTML: '<a href="mailto:me@example.com">email me@example.com</a>',
        verify: false
    });

    Prompter {overlay: null, storeURL: "https://marketplace.mozilla.org/app/myapp", supportHTML: "<a href="mailto:me@example.com">email me@example.com</a>", allowNoInstall: false, ignoreInternalError: false…}