Change default api client request body serialization

[nikhilym]

Description

This commit makes a change in the default api client - request body
serialization. The earlier code has a conversion for the request
body to convert to json before triggering the APIs, without checking
the content type of the request. This would make incorrect API calls
if the request body is not of type JSON. This commit checks if the
request body content is of type json and only convert it. If not,
then send in the body unconverted, leaving the initial input as is.

Motivation and Context

To allow default api client, to call Alexa-APIs that has other than application/json body content

Testing

Corresponding tests has been added to the default api client test suite. tox tests are successful.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.