Propagate request headers and standard headers to upstream services

[thovid]

• Composer should act like a proxy so that the appropriate subset of request headers is propagated to upstream
  ** see https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-14#section-7.1.3 for a definition of hop-by-hop headers that must be removed by a proxy
  ** see https://www.mnot.net/blog/2011/07/11/what_proxies_must_do on how a proxy should behave
• additional composer related headers should be propagated to upstream
  ** add x-forwarded-* / forwarded headers, see https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Forwarded
  ** add originally requested path

[thovid]

Closed by PR #35 - for the forwarded headers, #36 was opened