fix(iot-dev): Fix default Content Type and Content Encoding

[nabinbhandari]

Set default Content Type of D2C Message to null and default Content Encoding to UTF-8.

Github issue (fix#118)

Checklist

• I have read the [contribution guidelines] (https://github.com/Azure/azure-iot-sdk-java/blob/main/.github/CONTRIBUTING.md).
• I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• I submitted this PR against the correct branch:
  • This pull-request is submitted against the main branch.

Reference/Link to the issue solved with this PR (if any)

#118

Description of the problem

The existing code set the default content type (instead of content encoding) to "UTF-8". This would prevent using message routing by querying on the message body even if the content type was explicitly set to "application/json".

Description of the solution

Used default IoT Hub message charset name to set the content encoding of the message instead of content type. An application using the client SDK should still explicitly set the content type to "application/json" to enable proper message routing.