Make the ApiClient use application/x-www-form-urlencoded as content type

[supherman]

Summary

One of the issues I encountered after upgrading to V2 is that all of the tests that were stubbing requests to twilio failed, turns out Webmock does not support matching the request body for multipart/form-data requests, the error thrown was this:

ArgumentError: WebMock does not support matching body for multipart/form-data requests yet :(

I believe multipart/form-data is mostly used for file uploads, so, perhaps using application/x-www-form-urlencoded makes more sense to submit plain form data.

Testing

All Tests are passing, added body matching to the Twilio request stub.

Checklist

• Code follows the project's coding standards
• Tests have been added or updated to cover the changes
• Documentation has been updated (if applicable)
• All existing tests pass
• Conforms to the contributing guidelines

[excid3]

I think this is what I intended originally but wasn't paying close attention to it.

Chances are you won't be uploading files in notifications and you can always implement your own POST request if you did want to, so this makes sense to me to update.