=hco,str small cleanups
+hco introduce Uri.Host.inetAddresses for easy access to the actual addresses
=hco more accurate result types for HttpEntity.apply overloads
+hco introduce generic abstract accessor for HttpCredentials parameters
!hco more stringent following of plurally named container object

 - ContentDispositionType => ContentDispositionTypes
 - TransferEncoding => TransferEncodings
 - RangeUnit => RangeUnits
!hco get rid of extra HttpResponse constructor which changes ordering

The problem is that those extra constructors can easily lead to
ambiguities in overloading resolution. We should rely on named parameters
to get a similar effect than those extra constructors.
!hco #15392 use `params` instead of `parameters` consistently
!hco privatize some internal Uri.Host constructors
Merge pull request #15391 from spray/w/small-http-core-changes

+hco Small akka-http-core changes as a preparation for the Java API
=hco Add ResponseParserSpec, improve message parser
=hco Port over request rendering from spray-can
=hco Make Server header fully configurable (now consistent to User-Agent header on the client side)
!str Extend StreamIO API with `idleTimeout` config setting, improve `connectTimeout` model (refs #15401)
!hco Implement connection-level client API, refactor and strip down client and server infrastructure

Remove everything that is out of scope for akka-http-core M1.
Things like the host- and request-level client-side API will be added back later.
=hco Add ClientServerSpec and TestClient
=hco Remove `Low-Level API Design.rst`