! can: make Content-Length a long value, fixes #443

API changes:
  * `Content-Length`.length is now a Long
  * `max-content-length` setting is now also a Long
  * `incoming-auto-chunking-threshold-size` is now also a Long

This change makes only sense in combination with (auto-)chunking,
because we are not able to support byte arrays (HttpEntities) bigger
than Int.MaxValue.
Merge pull request #456 from jrudolph/i/443/long-content-length

! can: make Content-Length a long value, fixes #443