Template scripts fail to run if "head script" is included with "defer"

[panasyuk]

There is a script in

spree_braintree_vzero/app/views/spree/braintree_vzero/_paypal_checkout.html.erb

Line 13 in 684b5ba

SpreeBraintreeVzero.checkoutFormId = '#update-cart';

which initializes "PayPal Checkout" button on cart page. It fails with ReferenceError: Can't find variable: SpreeBraintreeVzero error if application's JS is included in head tag in deferred mode:

<%= javascript_include_tag 'spree/frontend/all', defer: true %>

That happens because the head script runs after DOM is loaded causing head JS to run after the template's script.

The same works for another script: https://github.com/spree-contrib/spree_braintree_vzero/blob/master/app/views/spree/checkout/payment/braintree_vzero/_three_d_secure.html.erb and it causes errors on Payment step.

[panasyuk]

The issues are probably related to the problem:
#220
#83