!str switch to a stable `Source.empty` val
=htc small fix and improvements to HttpEntity
!htc #15674 refactor, improve Multipart model
!htp #15674 upgrade to new Multipart model
+htp #15930 import RangeDirectives from spray
=htc fix exception message (thanks, 2.11.x warnings!)
+htp #15922 import FileAndResourceDirectives from spray