Replace endRequest with beginBody

Check if newer `beginBody` method exists in [ArduinoHTTPClient](https://github.com/arduino-libraries/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();
```

Start at src/AdafruitIO_Group.cpp around line 322 and check whether the current ArduinoHTTPClient provides beginBody. Replace the endRequest call when the dependency supports it, then test the change on the relevant hardware targets and confirm requests still work.

Written by the indexing model from the issue text.