Improve LittleFS support (#7071)

Previously, when the path was a directory, but didn't have a slash at
the end, a 0 byte response would be sent when using LittleFS. Now, it
will return a 404, like was always the case when using SPIFFS.

Author: JasperHorn

libraries/ESP8266WebServer/src/detail/RequestHandlersImpl.h

@@ -80,7 +80,7 @@ class StaticRequestHandler : public RequestHandler<ServerType> {
         else {
             _isFile = false;
         }
-        
+
         DEBUGV("StaticRequestHandler: path=%s uri=%s isFile=%d, cache_header=%s\r\n", path, uri, _isFile, cache_header);
         _baseUriLength = _uri.length();
     }
@@ -134,6 +134,11 @@ class StaticRequestHandler : public RequestHandler<ServerType> {
         if (!f)
             return false;
 
+        if (!f.isFile()) {
+            f.close();
+            return false;
+        }
+
         if (_cache_header.length() != 0)
             server.sendHeader("Cache-Control", _cache_header);