makeHeaders refactoring

[Esen Dzhailobaev]

Having to iterate over object to create Headers of them is obsolete.
Headers constructor accepts a plain object and constructs Headers class instance of it.

spec

const makeHeaders = obj => { let h = new Headers() for (let key in obj) { h.append(key, obj[key]) } return h }

becomes

const makeHeaders = obj => new Headers(obj)