Bug fix (#45)

* HTTP 302 and 304 Support

Add support for http redirection (302) and http not modified (304) to
reduce the load the server.
server.redirect(“url”, “location”, exclude-ip) will respond with 302 to
redirect the browser to a different url, this is useful for backward
compatibility and to redirect call to CDN when not no AP mode.
server.serveStatic has a new optional parameter to get the
Last-Modified date for all files serve for this location, when the
browser request have the same If-Modified-Since header value, the
server respond with 304 code instead of serving the file.

* First round of performance improvements.

* Merge remote-tracking branch 'me-no-dev/master' into performance

# Conflicts:
#	src/WebHandlerImpl.h
#	src/WebHandlers.cpp

* use of sprintf

* Remove sections not related.

* Fix a bug to serve a file in directory (e.g. serverStatic("/", SPIFFS, "/index.html");

src/WebRequest.cpp

@@ -117,7 +117,7 @@ void AsyncWebServerRequest::_onData(void *buf, size_t len){
       _temp.concat(str);
       _temp.trim();
       _parseLine();
-      if (++i < len) _onData(buf+i, len-i); // Still have more buffer to process
+      if (++i < len) _onData(str+i, len-i); // Still have more buffer to process
     }
   } else if(_parseState == PARSE_REQ_BODY){
     if(_isMultipart){