fix: use Base64.strict_encode64 instead of Base64.encode64

[yujideveloper]

Fixes

When I used Base64.encode64 to encode attachments, I got the following error:

{
  "errors": [
    {
      "message": "The attachment content must be base64 encoded.",
      "field": "attachments.0.content",
      "help": "http://sendgrid.com/docs/API_Reference/Web_API_v3/Mail/errors.html#message.attachments.content"
    }
  ]
}

It is resolved by changing to encode attachments using Base64.strict_encode64 where no line feeds are added.

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 worksx
• 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.