How to set Host header with LoadBalancing

According to #560 you may be discussing but I got confused when using Finagle's load balancing feature. In order to distribute requests to multiple servers, we instantiate a service via Http.Client#newService as follows,

val client = Http.client.newService("hostA.org:80, hostB.org:80")

Then construct request,

val request = Request("/path/to/service")
request.host = ???

client(request)

What should we fill ??? with?

Moreover, we're forced to pass the host addresses twice when the service creation and the building request. It's not DRY.

Are there any reasons that the Host header automatically set that given to the Service?