Characteristic 'Heating Threshold Temperature' Throws A Warning When Setting To 4.5 Celsius
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**Describe The Bug:**
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Nest sets safety temperature to 4.5 Celsius which is a valid temperature. However, HomeBridge still sends a warning says it's illegal because it's below 9. HomeBridge should check whether it's Celsius or Fahrenheit then check the value. 

**To Reproduce:**
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Use Celsius in IOS.

**Expected behavior:**
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Successfully sets safety temperature to 4.5 Celsius which is a valid temperature. 

[**Logs:**]()
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[homebridge-nest] This plugin generated a warning from the characteristic 'Heating Threshold Temperature': characteristic was supplied illegal value: number 4.5 exceeded minimum of 9. See https://git.io/JtMGR for more info.
