Context.Scheme accepts malformed forwarded scheme values used by host redirects

### Issue Description `Context.Scheme()` is documented as returning `http` or `https`, but `X-Forwarded-Proto`, `X-Forwarded-Protocol`, and `X-Url-Scheme` values are returned as-is. This affects `middleware.WWWRedirect()` and `middleware.NonWWWRedirect()`, since both use `c.Scheme()` when building the `Location` header. Example: ```http Host: app.example X-Forwarded-Proto: //external.example ``` With `middleware.WWWRedirect()`, the response contains: ```http Location: //external.example://www.app.example/ ``` Because the value starts with `//`, browsers resolve the `Location` as a network-path reference with host `external.example`. This requires the redirect middleware to be enabled and the forwarded header to reach Echo. `Context.Scheme()` should ignore forwarded scheme values other than `http` and `https`, matching its documented return values. ### Relevant code In `context.go`: ```go if scheme := c.request.Header.Get(HeaderXForwardedProto); scheme != "" { return scheme } if scheme := c.request.Header.Get(HeaderXForwardedProtocol); scheme != "" { return scheme } if scheme := c.request.Header.Get(HeaderXUrlScheme); scheme != "" { return scheme } ``` In `middleware/redirect.go`: ```go return true, scheme + "://www." + host + uri return true, scheme + "://" + host[4:] + uri ``` ### Version/commit This appears to affect both the current v5 branch and the supported v4 branch.