Allow to override "User-Agent" header for client-side with application.conf

[apehovski]

Custom value for key "akka.http.client.user-agent-header" currently does not override default value.
As I see, also "akka.http.parsing" values affected.

Minor pull-request:
#1493

[jrudolph]

There's indeed a problem here but I'd say it's a bit different:

• currently, for pool usages, akka.http.host-connection-pool.client settings override the general client settings but fall back when missing (which is how it should be)
• however, reference.conf contains a copy of the client settings (which we introduced, I think, to make the reference.conf more readable). That means, however, that akka.http.client settings never apply to the pool because we provide this copy in the reference.conf.

In fact, iirc, this whole setup was only done, to be able to provide a self-contained version of host-connection-pool settings which needs to wrap a set of client settings. Having to create self-contained versions of these settings seems to be a rare case, though.

The solution seems to be, to remove all concrete settings from akka.http.host-connection-pool.client, so that in general you can set settings just in akka.http.client and only have to use akka.http.host-connection-pool.client in special cases.

akka.http.{client,server}.parsing already uses the same style.