Fix gemini gateway proxying to work with gateways like tildeverse's

src/gmi/gemtext.go

@@ -7,7 +7,6 @@ import (
 	"io"
 	"net/url"
 	"path"
-	"path/filepath"
 	"regexp"
 	"strings"
 )
@@ -50,11 +49,10 @@ func GemtextToMarkdown(dst io.Writer, src io.Reader, gmiGateway *url.URL) error
 			}
 
 			if u.Scheme == "gemini" && gmiGateway != nil {
-
-				newU := *gmiGateway
-				newU.Path = filepath.Join(newU.Path, u.Host, u.Path)
-				newU.RawQuery = u.RawQuery
-				u = &newU
+				newUStr := gmiGateway.String() + u.Host + u.Path
+				if u, err = url.Parse(newUStr); err != nil {
+					return fmt.Errorf("parsing proxied URL %q: %w", newUStr, err)
+				}
 			}
 
 			isImg := hasImgExt(u.Path)