Fix basicauth for credentials requiring percent encoding

[glynnbird]

Overview

As reported in #363, when the CouchDB URL includes a username & password and those credentials contain characters that need to be "percent encoded", then Nano's auth failed. (It did work fine for session auth).

After investigation, it turns out that new URL, the Node.js URL parser returns the username and password strings percent encoded. This meant that when these values were base64 encoded to become an Authorization header, they contained the wrong data (but only for usernames and passwords with "special characters"). See https://nodejs.org/api/url.html#urlusername & https://nodejs.org/api/url.html#urlpassword.

The solution is simple: just decodeURIComponent the values before base64 encoding them.

Testing recommendations

Automated test added.

GitHub issue number

Fixes issue #363

Checklist

• Code is written and works correctly;
• Changes are covered by tests;