[DHT22] modified interpretation of temperature values in byte 2 and 3, such that negative temperature values are handled correctly

[bkormann80]

This pull request solves the issue described in #221.

Negative temperature values from a DHT22 sensor can be interpreted as a 16 bit signed integer (short). There is no additional differentiation needed.

float DHT::readTemperature(bool S, bool force) {
  float f = NAN;
  short s = 0;
...
    case DHT22:
      s = ((short) data[2]) << 8 | data[3];
      f = s * 0.1;
      if (S) {
        f = convertCtoF(f);
      }
      break;
... 

The patch was successfully tested using five different DHT22 sensors on an Arduino UNO and an ESP8266 at positive and negative temperatures.