Fixes the opening of the auth url in the browser in .NetCore

Explicitly set UseShellExecute to true, because in .NetFramework the default value is true but in .NetCore the default value changed to false
See: dotnet/winforms#1520 (comment)
and https://github.com/dotnet/corefx/issues/24704

SpotifyNet/WebAuthorization.cs

@@ -45,7 +45,15 @@ public string GetAuthorizationCode(string client_id, string redirect_uri, Scope
 
             using (var server = new Webserver(redirect_uri))
             {
-                Process.Start(uri.ToString());
+                Process.Start(new ProcessStartInfo(uri.ToString())
+                {
+                    // Explicitly set UseShellExecute to true, 
+                    // because in .NetFramework the default value is true but in
+                    // .NetCore the default value changed to false
+                    // See: https://github.com/dotnet/winforms/issues/1520#issuecomment-515899341
+                    // and https://github.com/dotnet/corefx/issues/24704
+                    UseShellExecute = true
+                });
                 result = server.WaitListen();
             }