S3: Fix encoding when generating public urls of S3 objects.

This code feels worryingly stringily typed, but using URI does not help
because the uriPath still has to be constructed with the right
uri-encoding.

doc/bugs/versioned_S3_generates_invalid_encoded_url.mdwn

@@ -17,3 +17,5 @@ frustratingly vague.
 
 Experimentally, even url-encoding alphanumerics works in these urls.
 So it would be ok to use a standard URI encoder.
+
+> [[fixed|done]] --[[Joey]]