http status code 504 for upstream service timeout (#1977)
http status code 504 for upstream service timeout (#1977)

resolves #1975

When `Enforced Timeout` plugin is enabled, the gateway response status
code indicates a client error `408 Request Timeout`.

In fact, there is nothing wrong with the client request and this is
misleading.

The correct http status code for this is `504 - Gateway Timeout`.

> The server was acting as a gateway or proxy and did not receive a
timely response from the upstream server.
lonelycode/tyk,lonelycode/tyk,lonelycode/tyk