When generating the url for a mounted engine through its proxy (e.g. foo.bar_path), the relative_url_root should not be used when generating the proxy’s url prefix.

(fixes rails#20920)

When generating the url for a mounted engine through its proxy, the path should be the sum of three parts:

1. Any `SCRIPT_NAME` request header or the value of `relative_url_root`.
2. A prefix (the engine's mounted path).
3. The path of the named route inside the engine.

Since commit rails@0703453, this has been broken. Instead, the path is the sum of four parts:

1. Any `SCRIPT_NAME` request header or the value of `relative_url_root`.
2. The value of `relative_url_root`.
3. A prefix (the engine's mounted path).
4. The path of the named route inside the engine.

This commit fixes the regression by providing a benign truthy value for `options[:script_name]` (an empty string) when the prefix is being generated, avoiding the duplicate `relative_url_root` value in the final result.

This resolves rails#20920, resolves rails#21459, and closes rails#20958

When generating the url for a mounted engine through its proxy, the path should be the sum of three parts:

1. Any `SCRIPT_NAME` request header or the value of `relative_url_root`.
2. A prefix (the engine's mounted path).
3. The path of the named route inside the engine.

Since commit rails@44ff031, this has been broken. Instead, the path is the sum of four parts:

1. Any `SCRIPT_NAME` request header or the value of `relative_url_root`.
2. The value of `relative_url_root`.
3. A prefix (the engine's mounted path).
4. The path of the named route inside the engine.

This commit fixes the regression by providing a benign truthy value for `options[:script_name]` (an empty string) when the prefix is being generated, avoiding the duplicate `relative_url_root` value in the final result.

This resolves rails#20920, resolves rails#21459, and closes rails#20958

When generating the url for a mounted engine through its proxy, the path should be the sum of three parts:

1. Any `SCRIPT_NAME` request header or the value of `ActionDispatch::Routing::RouteSet#relative_url_root`.
2. A prefix (the engine's mounted path).
3. The path of the named route inside the engine.

Since commit 44ff031, this has been broken. Step 2 has been changed to:

2. A prefix (the value of `ActionDispatch::Routing::RouteSet#relative_url_root` + the engine's mounted path).

The value of `ActionDispatch::Routing::RouteSet#relative_url_root` is taken into account in step 1 of the route generation and should be ignored when generating the mounted engine's prefix in step 2.

This commit fixes the regression by having `ActionDispatch::Routing::RouteSet#url_for` check `options[:relative_url_root]` before falling back to `ActionDispatch::Routing::RouteSet#relative_url_root`. The prefix generating code then sets `options[:relative_url_root]` to an empty string. This empty string is used instead of `ActionDispatch::Routing::RouteSet#relative_url_root` and avoids the duplicate `relative_url_root` value in the final result.

This resolves #20920 and resolves #21459
(cherry picked from commit bcfbd8b)