This is a terrible hack, bound to be incompatible with future versions of the framework and so on.
But it works!
(on my machine…)
Uri uri = new Uri("http://example.com/%2F");
ForceCanonicalPathAndQuery(uri);
using (WebClient webClient = new WebClient())
{
webClient.DownloadData(uri);
}
void ForceCanonicalPathAndQuery(Uri uri){
string paq = uri.PathAndQuery; // need to access PathAndQuery
FieldInfo flagsFieldInfo = typeof(Uri).GetField("m_Flags", BindingFlags.Instance | BindingFlags.NonPublic);
ulong flags = (ulong) flagsFieldInfo.GetValue(uri);
flags &= ~((ulong) 0x30); // Flags.PathNotCanonical|Flags.QueryNotCanonical
flagsFieldInfo.SetValue(uri, flags);
}