Tests: Remove HttpClient to only use one HTTP client implementation

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 used for some plugin tests 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.