!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
Merge pull request #16183 from spray/wip-15674-mathias

Refactor, improve Multipart model
+htc #16084 introduce CustomHeader superclass for user-defined headers