Preserve host header and allow route specific configuration about gateleen

[hiddenalpha]

Edit: There we go -> #397

WARN: The Host header has lots of behavior specified by the HTTP protocol. So we need to take care when implementing some behavior here. See eg rfc7230_section_5.4, rfc2616_sec14.23, rfc2616_sec5.2, rfc2616_sec19.6.1.1.

We MUST make sure we do not violate any of them if we want gateleen to be compatible to HTTP.

For example. The suggested behavior:

2. Fill in target host:post if Host header is missing

IMHO is invalid. As HTTP/1.1 says:

A server MUST respond with a 400 (Bad Request) status code to any
HTTP/1.1 request message that lacks a Host header field

Therefore filling this header would only be allowed for a HTTP/1.0 (or older) request.

So given our request is a HTTP/1.1 request, we should respond 400 as silently put in a header would violate HTTP/1.1.

Some (randomly picked by me) citations for example:

When a proxy receives a request with an absolute-form of
request-target, the proxy MUST ignore the received Host header field
(if any) and instead replace it with the host information of the
request-target

A proxy that forwards such a request MUST generate a
new Host field-value based on the received request-target rather than
forward the received Host field-value

A server MUST respond with a 400 (Bad Request) status code to any
HTTP/1.1 request message that lacks a Host header field and to any
request message that contains more than one Host header field or a
Host header field with an invalid field-value

[Kusig]

Yep, this is mostly known but not really treated today by Gateleen anyhow. It simply just overwrites any Host header with the target host which sounds rather strange.

However, I will not remove this but just use it as fallback if there is no incoming host (from Proxy or Browser) or if there is no specific host in the route configured.

The assumption is that most of the backends behind gateleen just don't care about this Host:xxx heaeder currently. That's why this never popped up so far.

[mcweba]

Released in v1.1.77