Because of git problems, start from a new fork and create a new PR. This was PR esp8266#6457

Author: Jeroen88

libraries/ESP8266HTTPClient/examples/ReuseConnectionV2/ReuseConnectionV2.ino

@@ -0,0 +1,76 @@
+/**
+   reuseConnectionV2.ino
+
+    Created on: 22.11.2015
+
+   This example reuses the http connection and also restores the connection if the connection is lost
+*/
+
+
+#include <Arduino.h>
+
+#include <ESP8266WiFi.h>
+#include <ESP8266WiFiMulti.h>
+
+#include <ESP8266HTTPClient.h>
+
+ESP8266WiFiMulti WiFiMulti;
+
+HTTPClient http;
+WiFiClient client;
+
+void setup() {
+
+  Serial.begin(115200);
+  // Serial.setDebugOutput(true);
+
+  Serial.println();
+  Serial.println();
+  Serial.println("Connecting to WiFi...");
+
+  WiFi.mode(WIFI_STA);
+  WiFiMulti.addAP("SSID", "PASSWORD");
+
+  // wait for WiFi connection
+  while ((WiFiMulti.run() != WL_CONNECTED)) {
+    Serial.write('.');
+    delay(500);
+  }
+  Serial.println(" connected to WiFi");
+
+  // allow reuse (if server supports it)
+  http.setReuse(true);
+
+
+  http.begin(client, "http://jigsaw.w3.org/HTTP/connection.html");
+  //http.begin(client, "jigsaw.w3.org", 80, "/HTTP/connection.html");
+}
+
+void loop() {
+  for (int i = 0; i < 10; i++) {
+    Serial.printf("Reuse connection example, GET url for the %d time\n", i + 1);
+    int httpCode = http.GET();
+    if (httpCode > 0) {
+      Serial.printf("[HTTP] GET... code: %d\n", httpCode);
+
+      // file found at server
+      if (httpCode == HTTP_CODE_OK) {
+        http.writeToStream(&Serial);
+      }
+    } else {
+      Serial.printf("[HTTP] GET... failed, error: %s\n", http.errorToString(httpCode).c_str());
+      // Something went wrong with the connection, try to reconnect
+      http.end();
+      http.begin(client, "http://jigsaw.w3.org/HTTP/connection.html");
+      //http.begin(client, "jigsaw.w3.org", 80, "/HTTP/connection.html");
+    }
+
+    Serial.println("\n\n\nWait 5 second...\n");
+    delay(5000);
+  }
+
+  http.end();
+
+  Serial.println("Done testing, now wait forever");
+  for (;;) delay(100); // Wait forever
+}

libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp

@@ -456,6 +456,10 @@ void HTTPClient::disconnect(bool preserveClient)
 #endif
         }
     } else {
+        if (!preserveClient && _client) { // Also destroy _client if not connected()
+            _client = nullptr;
+        }
+
         DEBUG_HTTPCLIENT("[HTTP-Client][end] tcp is closed\n");
     }
 }
@@ -922,7 +926,9 @@ int HTTPClient::writeToStream(Stream * stream)
         return returnError(HTTPC_ERROR_NO_STREAM);
     }
 
-    if(!connected()) {
+    // Only return error if not connected and no data available, because otherwise ::getString() will return an error instead of an empty
+    // string when the server returned a http code 204 (no content)
+    if(!connected() && _transferEncoding != HTTPC_TE_IDENTITY && _size > 0) {
         return returnError(HTTPC_ERROR_NOT_CONNECTED);
     }
 
@@ -931,11 +937,13 @@ int HTTPClient::writeToStream(Stream * stream)
     int ret = 0;
 
     if(_transferEncoding == HTTPC_TE_IDENTITY) {
-        ret = writeToStreamDataBlock(stream, len);
+        if(len > 0) {
+            ret = writeToStreamDataBlock(stream, len);
 
-        // have we an error?
-        if(ret < 0) {
-            return returnError(ret);
+            // have we an error?
+            if(ret < 0) {
+                return returnError(ret);
+            }
         }
     } else if(_transferEncoding == HTTPC_TE_CHUNKED) {
         int size = 0;
@@ -1289,6 +1297,7 @@ int HTTPClient::handleHeaderResponse()
                     _canReuse = (headerLine[sizeof "HTTP/1." - 1] != '0');
                 }
                 _returnCode = headerLine.substring(9, headerLine.indexOf(' ', 9)).toInt();
+                _canReuse = (_returnCode != '0');
             } else if(headerLine.indexOf(':')) {
                 String headerName = headerLine.substring(0, headerLine.indexOf(':'));
                 String headerValue = headerLine.substring(headerLine.indexOf(':') + 1);