Fix Host header for proxy client requests

When using `ban` requests with Varnish proxy client, requests are queued
and sent with `flush()`. Queued requests are registered without host,
making Guzzle create an empty `Host` header.

When flushing, Guzzle request is re-created starting
with the original one (so without host), with all registered headers.
The problem is that the empty `Host` header is also copied, and thus not
re-created by Guzzle with the server URL. In the backend, cURL sees it
and takes it into account, overriding passed URL silently.

This causes requests with empty host being fired by Guzzle, which is
kind of annoying :smiley:.