= can: don't close connection with chunkless streaming and explicit Content-Length, fixes #783

Connection headers are now only rendered if needed for the
protocol version, i.e. `Connection: close` only in HTTP/1.1 and
`Connection: keep-alive` only in HTTP/1.0.
Merge pull request #802 from jrudolph/i/783-dont-close-with-explicit-content-length

= can: don't close connection with chunkless streaming and explicit Content-Length, fixes #783