+hco introduce HttpEntity.Strict for strict ByteString data
=hco chunkless streaming has left the building

It can always be replaced by either HttpEntity.Default (explicit
content-length given) or HttpEntity.CloseDelimited (no content-length
given).
=hco restructure renderer tests and add check for data size against content-length of HttpEntity.Default
=hco emit Strict entity if parser already has complete entity data available
=hco add unified `dataBytes` accessor to HttpEntity
=hco add HttpEntity.toStrict + tests
=str preliminary fix to #15353
Merge pull request #3 from spray/w/introduce-Strict-entity

Introduce HttpEntity.Strict and other smaller changes