Core - ResourceHandler set Stream.Position = 0 and if CanSeek

- Prior to version 75 the Stream.Position was set to 0 before the ResourceHandler would read the Stream.
This restores that behaviour.
- Minor refactor of if statement for Stream.Can seek, only override ResponseLength with Stream.Length of not already set

Resolves #2903

CefSharp/ResourceHandler.cs

@@ -202,10 +202,17 @@ void IResourceHandler.GetResponseHeaders(IResponse response, out long responseLe
             {
                 responseLength = ResponseLength.Value;
             }
-            else if (Stream != null && Stream.CanSeek)
+
+            if (Stream != null && Stream.CanSeek)
             {
-                //If no ResponseLength provided then attempt to infer the length
-                responseLength = Stream.Length;
+                //ResponseLength property has higher precedence over Stream.Length
+                if (ResponseLength == null || responseLength == 0)
+                {
+                    //If no ResponseLength provided then attempt to infer the length
+                    responseLength = Stream.Length;
+                }
+
+                Stream.Position = 0;
             };
         }