fix: serialize mail object before client dispatch

[matiasyocca]

Fixes

This PR fixes the call to Sendgrid\Client::makeRequest() within the BaseSendGridClientInterface::send() method. This was caused by the v4.1.2 update to sendgrid/php-http-client mentioned in sendgrid/php-http-client#170 and would fix that issue.

Current test suite covers this fix by ensuring the call to that method works, so no new test cases should be needed.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• 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

[matiasyocca]

Closing as fixed by sendgrid/php-http-client#171