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#### Describe the bug
In situations that use Basic auth and do not pass the parameter "password" _**requestHeaders.Authorization**_ is concatenated unduly "**: undefined**".

#### To Reproduce
Any request using **Basic Authentication** without passing the parameter "**password**" as an example below.

```js
var axios = require("axios")
const instance = axios.create();
instance.request({
    method: 'POST',
    baseURL: ".../XPTO",
    auth: { username: "janedoe" }}
    ).then(
        // do something
    );
```

#### Expected behavior
To maintain compatibility with previous versions, the proper behavior would be to test if the parameter "**password**" was passed, otherwise concatenate empty.

#### Environment
 - Axios Version [e.g. 0.20.0]
 - Adapter [e.g. XHR/HTTP]
 - Browser [e.g. Chrome, Safari]
 - Browser Version [e.g. 22]
 - Node.js Version [e.g. 13.0.1]
 - OS: [e.g. iOS 12.1.0, OSX 10.13.4]
 - Additional Library Versions [e.g. React 16.7, React Native 0.58.0]

#### Additional context/Screenshots
A problem arising from the lack of compatibility with previous versions is to force all current projects that do not pass a "password" parameter to have to include this parameter with an empty value.
lib/adapters/xhr.js
'use strict';
var utils = require('./../utils');
var settle = require('./../core/settle');
var cookies = require('./../helpers/cookies');
var buildURL = require('./../helpers/buildURL');
var buildFullPath = require('../core/buildFullPath');