title: Custom HTTP Headers with old proxies
body: <p>Is it true that some old proxies/caches will not honor some custom HTTP headers? If so, can you prove it with sections from the HTTP spec or some other information online?</p>
  
  <p>I'm designing a REST API interface. For versioning I'm debating whether to use version as a part of the URL like (/path1/path2/v1 OR /path1/path2?ver=1) OR to use a custom Accepts X-Version header. </p>