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

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• [x] I have added tests that prove my fix is effective or that my feature worksx
• [x] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

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