Properly handle prefix="" in CustomProvider()

It might be that an app could support either running behind a reverse
proxy or directly. In the latter case, prefix would be "". This modifies
CustomProvider to properly handle that case.

Previously, CustomProvider() would apply this to prefix unconditionally:

	prefix = "/" + strings.Trim(prefix, "/")

Starting with prefix = "", prefix would become "/", making appWASM
"//web/app.wasm". A browser that acted on such a path might erroneously
load, e.g., "http://web/app.wasm".

Now, CustomProvider() only trims and concatenates when prefix != "".

Signed-off-by: W. Michael Petullo <mike@flyn.org>

pkg/app/resource.go

@@ -96,7 +96,10 @@ func GitHubPages(repoName string) ResourceProvider {
 // given prefix.
 func CustomProvider(path, prefix string) ResourceProvider {
  root := strings.Trim(path, "/")
- prefix = "/" + strings.Trim(prefix, "/")
+
+ if prefix != "" {
+ prefix = "/" + strings.Trim(prefix, "/")
+ }
 
  return localDir{
  Handler: http.FileServer(http.Dir(root)),