Allow adding basic authentication to data adapter in URL

Adding basic authentication credentials to the URL of an HTTP JSONPath data adapter does not work as expected, as the username and password are ignored used in the URL and no `Authorization` header is generated from them.

This means that to use basic authentication, the user must calculate the `Authorization` header manually and set it up in the HTTP headers.

## Expected Behaviour
Using a URL with credentials in it should generate the `Authorization` header.

## Current Behaviour
Using a URL with credentials does not generate an `Authorization` header.

## Steps to Reproduce (for bugs)
1. Create HTTP JSONPath data adapter
2. Add a URL with basic authentication credentials, e.g. `https://username:password@myserver.com?key=${key}`
3. Testing the data adapter fails with a 401 error, no information is retrieved from the endpoint

## Your Environment
* Graylog Version: 2.4.6, 3.0.0-alpha.4-SNAPSHOT