This enables customizing HTTP headers which adds some extra flexibility.
This does not break anything of course because this change introduces a new constructor with a new additional HTTPClient argument for HTTPUpdate class.
* Fix error in PR #2048: if ::available() is called before ::connect() _rxBuffer is not initialised
* Fixed flash size check and added SPIFFS size check
* Rewriting ESP.getFreeSketchSpace(), moving code from HTTPUpdate.cpp
* Added HTTPUpdate class for downloading sketches from a server
* Added HTTPUpdate class for downloading sketches from a server
* Added HTTPUpdate to CMakeLists.txt
* Change ESP8266 class references to ESP32 for httpUpdate.ino example
* Change ESP8266 class references to ESP32 for httpUpdate.ino example. setLedPin() commented out because not all boards support LED_BUITLIN
* Added check to handle mixup of old and present api properly
* Correct HTTPClient::setTimeout() to convert milliseconds to seconds. Correct WiFiClient::setTimeout() to call Stream::setTimeout() with seconds converted back to milliseconds. Remove inproper checks for _insecure.
* Added small comment because it looked like the Travis build did not finish
