Extend redirect URI validation with protocol check

[Rulox]

Proposed changes

• Update docs with proper variables
• Make sure that validation of Redirect URLs (both in action and error pages) check for the protocol of the URL to be a fully working URL (this is to avoid NGINX to try to open local files). Now a redirect URL must contain ://.

[Dean-Coakley]

Lgtm pending the above change

[pleshakov]

@Rulox thanks. a few notes about using $upstream_status instead of $status

[pleshakov]

@Rulox Thanks! The changes looks good!

Do we still support upstream_status in the error page redirect tho?

Not sure if this variable can be useful. The problem is that it could contain multiple values:

$upstream_status
keeps status code of the response obtained from the upstream server. Status codes of several responses are separated by commas and colons like addresses in the $upstream_addr variable. If a server cannot be selected, the variable keeps the 502 (Bad Gateway) status code.

Considering that, I don't think it could be useful in redirects.

[tellet]

lgtm