Replace endRequest with beginBody

[brentru]

Check if newer beginBody method exists in ArduinoHTTPClient. Replace endRequest and test on hardware targets.

https://github.com/adafruit/Adafruit_IO_Arduino/blob/master/src/AdafruitIO_Group.cpp#L322

  // the following call to endRequest
  // should be replaced by beginBody once the
  // Arduino HTTP Client Library is updated
  // _io->_http->beginBody();
  _io->_http->endRequest();