fix(http): decode proxy url auth

[Thore1954]

#4852 introduced a subtle bug that recently broke one of our apps. Here's the issue:
Node's url.parse automatically decodes the auth property:

import url from 'url'
const myURL = url.parse('http://user%3F:pass%3F@localhost:4000')
console.log(myURL.auth) // "user?:pass?"

However, the URL interface returns username and password undecoded:

const myURL = new URL('http://user%3F:pass%3F@localhost:4000')
console.log(myURL.username) // "user%3F"
console.log(myURL.password) // "pass%3F"

This PR reintroduces the old behavior by manually decoding the username and password properties with decodeURIComponent.