Fixes

Added middleware for aplication/json and corrected to_s() for request in case of json body

[x] I acknowledge that all my contributions will be made under the project's license
[x] I have made a material change to the repo (functionality, testing, spelling, grammar)
[x] I have read the Contribution Guidelines and my PR follows them
[x] I have titled the PR appropriately
[x] I have updated my branch with the main branch
[ ] I have added tests that prove my fix is effective or that my feature works
[ ] I have added the necessary documentation about the functionality in the appropriate .md file
[ ] I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

twilio / twilio-ruby