Fix IoT Hub messages to be JSON format and utf-8 encoded.

[RuinedStar]

Originally it was sending the content in base64 format to IoT Hub.
And the content is non human readable if we routed to Azure storage. (Fig. 1)
The pull request is to fix the content encoding from base64 to utf-8 and make it readable. (Fig. 2)

Fig.1

Fig.2

[MaxStrange]

Thanks Arthur!

[Devinwong]

Thanks for the quick turnaround, Arthur