Use os-related file-seperator in FileProvider

Author: simonwep

src/main/java/express/middleware/FileProvider.java

@@ -6,6 +6,7 @@
 import express.utils.Status;
 import express.utils.Utils;
 
+import java.io.File;
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -55,7 +56,7 @@ public void handle(Request req, Response res) {
             path = "index.html";
         }
 
-        Path reqFile = Paths.get(root + "\\" + path);
+        Path reqFile = Paths.get(root + File.separator + path);
 
         /*
          * If the file wasn't found, it will search in the target-directory for