Support for concatenation of headers (#4864)

If the server returns several headers of the same key (e.g Set-Cookie) only the last one is returned, causing issues in communicating with some servers where cookies are required.

This change concatenates the headers of the same key separated by "," to alleviate this issue.

Author: Mick Wheeler

Diff for: libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp

@@ -1041,8 +1041,13 @@ int HTTPClient::handleHeaderResponse()
 
                 for(size_t i = 0; i < _headerKeysCount; i++) {
                     if(_currentHeaders[i].key.equalsIgnoreCase(headerName)) {
-                        _currentHeaders[i].value = headerValue;
-                        break;
+                        if (_currentHeaders[i].value != "") {
+                            // Existing value, append this one with a comma
+                            _currentHeaders[i].value += "," + headerValue;
+                        } else {
+                            _currentHeaders[i].value = headerValue;
+                        }
+                        break; // We found a match, stop looking
                     }
                 }
             }