Once again, we allow nil values in response headers.  We've had
this bug since March 2009, and thus cannot expect existing
applications and middlewares running unicorn to fix this.

Unfortunately, supporting this bug contributes to application
server lock-in, but at least we'll document it as such.

Thanks to Owen Ou <o@heroku.com> for reporting this regression:

  http://bogomips.org/unicorn-public/CAO47=rJa=zRcLn_Xm4v2cHPr6c0UswaFC_omYFEH+baSxHOWKQ@mail.gmail.com/

Additionally, systemd examples are now in the examples/ directory
based on a post by Christos Trochalakis <yatiohi@ideopolis.gr>:

  http://bogomips.org/unicorn-public/20150708130821.GA1361@luke.ws.skroutz.gr/