fix: ctx.request.origin should respect proxy flag via X-Forwarded-Proto

[guoyangzhen]

Fixes #1746

Problem

With app.proxy = true, ctx.request.origin ignores X-Forwarded-Proto and always returns http://. This is because the getter returns this.req.headers.origin (the CORS Origin header) instead of constructing the origin from the request URL.

Meanwhile ctx.protocol correctly returns https from X-Forwarded-Proto, and ctx.host correctly reads X-Forwarded-Host.

Fix

Change origin getter to construct ${protocol}://${host} using the existing getters that already respect the proxy flag. Returns null when host is empty (maintaining backward compatibility for cases where no host header is present).

Test

With nginx proxy passing X-Forwarded-Proto: https and X-Forwarded-Host: mywebsite.com:

• Before: ctx.request.origin = http://mywebsite.com (from raw Origin header)
• After: ctx.request.origin = https://mywebsite.com (from proxy headers)

[guoyangzhen]

The CI failure is because this PR changes the semantics of ctx.origin:

Before: Returns the Origin HTTP request header (used for CORS)
After: Constructs protocol://host from the request URL

The test sets headers.origin = 'http://example.com' and headers.host = 'localhost'.

• Old behavior: returns http://example.com (the Origin header)
• New behavior: returns http://localhost (constructed from host)

These serve different purposes. The Origin header is used for CORS, while protocol://host is the request's actual URL origin. If the intent is to provide the request URL origin, consider adding a separate getter (e.g., request.urlOrigin) instead of changing the existing ctx.origin behavior, which would be a breaking change for CORS middleware.