Tests: Remove HttpClient to only use one Http client

The HTTP client implementation used by the Elasticsearch REST tests is
backed by apache http client instead of a self written helper class,
that uses HttpUrlConnection. This commit removes the old simple HttpClient
class and uses the more powerful and reliable one for all tests.

It also fixes a minor bug, that when sending a 301 redirect, a Location
header needs to be added as well, which was uncovered by the switching
to the new client.

Closes #7003