=htc small fix and improvements to HttpEntity
!htc #15674 refactor, improve Multipart model
!htp #15674 upgrade to new Multipart model
+str #16093 Add missing runWith variant
Merge pull request #16178 from akka/wip-16093-alternative-runWith-patriknw

+str #16093 Add missing runWith variant
Merge pull request #16183 from spray/wip-15674-mathias

Refactor, improve Multipart model
+htp #15930 import RangeDirectives from spray