!hco #15800 mark `Content-Length` header private for now

Instances will only created during HTTP message parsing and its value will
then be provided as a field of one of the HttpEntity subclasses.
=hco fail pending tests faster
=hco #15682 replace default toLowerCase calls with one using Locale.ROOT
Merge pull request #15826 from spray/w/fix-several-small-issues

Fixes for several small issues #15682, #15800
=str #15735 Prototype of FlowGraph

Add documentation