fix: propagate AllowHTTP to SourceURL() scheme selection

SourceURL() previously hardcoded https:// regardless of the AllowHTTP
config setting. This made testing with HTTP-only test servers impossible.

Add AllowHTTP field to ImageRequest and use it to determine the URL
scheme. The Service propagates the config setting to each request.

Fixes #1

internal/imgcache/sourceurl_test.go

@@ -18,9 +18,9 @@ func TestImageRequest_SourceURL_DefaultHTTPS(t *testing.T) {
 
 func TestImageRequest_SourceURL_AllowHTTP(t *testing.T) {
 	req := &ImageRequest{
-		SourceHost:  "localhost:8080",
+		SourceHost: "localhost:8080",
-		SourcePath:  "/photos/cat.jpg",
+		SourcePath: "/photos/cat.jpg",
-		AllowHTTP:   true,
+		AllowHTTP:  true,
 	}
 
 	got := req.SourceURL()