Add support for X-Forwarded-Proto to identify HTTPS protocol

[marian-kadanka]

Support X-Forwarded-Proto header since it is used in some proxy or
load balancer environments to identify HTTPS protocol - nor HTTPS,
nor SERVER_PORT variable is available in those scenarios to
identify the protocol used.

More info: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto

[nigrosimone]

before merging, please update readme.md with a new line into the changelog, eg.

:diamond_shape_with_a_dot_inside: Update | [Add support for X-Forwarded-Proto to identify HTTPS protocol](https://github.com/szepeviktor/w3-total-cache-fixed/pull/520) |

thanks for contributng to this project.

[marian-kadanka]

I've just added the line to the readme.md, thank you for the advice.

[nigrosimone]

Thanks, but please add the new line at the end of the list

[marian-kadanka]

Sorry, but there is the new line at the end of the list, should I add another one?

[nigrosimone]

Move your line at the end of the list. We add all change at the end of the list.

[marian-kadanka]

Ah, sorry, didn't get it... done.

[karlsoon]

does it is affects nginx?

[charlesLF]

@karlsoon : I just checked the commits in this PR, and it doesn't seem nginx was included.

There would need to be an additional "if" added in PgCache_Environment.php#1048, but I'm not a big fan of the use of "if's" in nginx (see If is Evil).

It'd be something like this:

		if ( $config->get_boolean( 'pgcache.cache.ssl' ) ) {
			$rules .= "set \$w3tc_ssl \"\";\n";

			$rules .= "if (\$http_x_forwarded_proto = https) {\n";
			$rules .= "    set \$w3tc_ssl _ssl;\n";
			$rules .= "}\n";

			$rules .= "if (\$scheme = https) {\n";
			$rules .= "    set \$w3tc_ssl _ssl;\n";
			$rules .= "}\n";

			$env_w3tc_ssl = "\$w3tc_ssl";
		}

Warning: I haven't tested it.

You can give it a try and let us know if it works.