fix #19 with proper struct unpack

tested with simulation:
# 0xE1 / 0xE2         dig_H2 [7:0]  / [15:8]    signed short
# 0xE3                dig_H3 [7:0]              unsigned char
# 0xE4 / 0xE5[3:0]    dig_H4 [11:4] / [3:0]     signed short
# 0xE5[7:4] / 0xE6    dig_H5 [3:0]  / [11:4]    signed short
# 0xE7                dig_H6                    signed char

>>> coeff = [ 0x12, 0xFF, 0xAA, 0xEE, 0x46, 0xCC, 0x88 ]
>>> coeff = list(struct.unpack('<hBbBbb', bytes(coeff)))
>>> float(coeff[0])                                 # 0xFF12 -> -238.0
-238.0
>>> float(coeff[1])                                 # 0xAA -> 170.0
170.0
>>> float((coeff[2] << 4) |  (coeff[3] & 0xF))      # 0xEE6 -> -282.0
-282.0
>>> float((coeff[4] << 4) | (coeff[3] >> 4))        # 0xCC4 -> -828.0
-828.0
>>> float(coeff[5])                                 # 0x88 -> -120.0
-120.0
>>>

Author: barbudor

adafruit_bme280.py

@@ -414,7 +414,7 @@ def _read_coefficients(self):
         self._humidity_calib = [0]*6
         self._humidity_calib[0] = self._read_byte(_BME280_REGISTER_DIG_H1)
         coeff = self._read_register(_BME280_REGISTER_DIG_H2, 7)
-        coeff = list(struct.unpack('<hBBBBb', bytes(coeff)))
+        coeff = list(struct.unpack('<hBbBbb', bytes(coeff)))
         self._humidity_calib[1] = float(coeff[0])
         self._humidity_calib[2] = float(coeff[1])
         self._humidity_calib[3] = float((coeff[2] << 4) |  (coeff[3] & 0xF))