set real "Last-Modified" header based on file's LastWrite time

Get file's LastWrite timestamp for file handlers (if supported by FS driver)
and construct proper "Last-Modified" header. Works fine for LittleFS.
If not supported by FS than fallback to previous implementation with manual value for "Last-Modified".

Signed-off-by: Emil Muratov <gpm@hotplug.ru>

Author: vortigont

README.md

@@ -876,8 +876,11 @@ handler->setCacheControl("max-age=30");
 ```
 
 ### Specifying Date-Modified header
-It is possible to specify Date-Modified header to enable the server to return Not-Modified (304) response for requests
-with "If-Modified-Since" header with the same value, instead of responding with the actual file content.
+Sever sets "Last-Modified" header automatically if FS driver supports file modification timestamps (LittleFS does).
+Server returns "Not-Modified" (304) response for requests with "If-Modified-Since" header with _the same_ value as file's mod date, instead of responding with the actual file content. It does not perform date calculations checking if File's mod date is newer or later than in "If-Modified-Since" header.
+
+For FS not supporting file timestamps (like deprecated SPIFFS) it is possible to specify Date-Modified header manually.
+
 ```cpp
 // Update the date modified string every time files are updated
 server.serveStatic("/", SPIFFS, "/www/").setLastModified("Mon, 20 Jun 2016 14:00:00 GMT");

src/WebHandlers.cpp

@@ -199,15 +199,29 @@ uint8_t AsyncStaticWebHandler::_countBits(const uint8_t value) const
 void AsyncStaticWebHandler::handleRequest(AsyncWebServerRequest *request)
 {
   // Get the filename from request->_tempObject and free it
-  String filename = String((char*)request->_tempObject);
+  String filename((char*)request->_tempObject);
   free(request->_tempObject);
   request->_tempObject = NULL;
   if((_username.length() && _password.length()) && !request->authenticate(_username.c_str(), _password.c_str()))
       return request->requestAuthentication();
 
   if (request->_tempFile == true) {
-    String etag = String(request->_tempFile.size());
-    if (_last_modified.length() && _last_modified == request->header(F("If-Modified-Since"))) {
+    time_t lw = request->_tempFile.getLastWrite();    // get last file mod time (if supported by FS)
+    if (lw) {
+      _last_modified.clear();
+      _last_modified.reserve(30);         // need 'Fri, 27 Jan 2023 15:50:27 GMT'
+      char *t = ctime(&lw);               // ctime 'Thu Jan 26 17:42:48 2023'
+      _last_modified.concat(t, 3);        // day of week
+      _last_modified.concat((char)0x2c);  // comma
+      _last_modified.concat(t+7, 3);      // day
+      _last_modified.concat(t+3, 4);      // month
+      _last_modified.concat(t+19, 5);     // year
+      _last_modified.concat(t+10, 9);     // time
+      _last_modified.concat(" GMT");
+      _last_modified.setCharAt(29, 0);    // null terminate
+    }
+    String etag(request->_tempFile.size());
+    if (_last_modified.length() && _last_modified == request->header("If-Modified-Since")) {
       request->_tempFile.close();
       request->send(304); // Not modified
     } else if (_cache_control.length() && request->hasHeader(F("If-None-Match")) && request->header(F("If-None-Match")).equals(etag)) {