#### Describe the bug
The irrelevant error returned when requesting illegal URL, like: "tel:484-695-3408"
```
TypeError: Cannot read property 'replace' of null
    at dispatchHttpRequest (node_modules/axios/lib/adapters/http.js:109:74)
    at new Promise (<anonymous>)
    at httpAdapter (node_modules/axios/lib/adapters/http.js:46:10)
    at dispatchRequest (node_modules/axios/lib/core/dispatchRequest.js:52:10)
```

#### To Reproduce
```js
axios.get('tel:484-695-3408').catch(err => console.error(err));
```

#### Expected behavior
Return descriptive error, like "Unsupported protocol tel:"

#### Environment
 - Axios Version: 0.21.1
 - Adapter: HTTP
 - Browser: none
 - Browser Version: none
 - Node.js Version: 12.19.0
 - OS: MacOS 10.14
 - Additional Library Versions: none

#### Additional context/Screenshots
none
lib/utils.js
forEach(b, function assignValue(val, key) {
    if (thisArg && typeof val === 'function') {
      a[key] = bind(val, thisArg);
    } else {
      a[key] = val;
    }
  });
  return a;
}
/**
 * Remove byte order marker. This catches EF BB BF (the UTF-8 BOM)
 *
 * @param {string} content with BOM
 * @return {string} content value without BOM
 */
function stripBOM(content) {