Detect if an attachment is Base64 encoded, if not, automatically encode it

[thinkingserious]

Issue Summary

This should be done in the [setContent](https://github.com/sendgrid/sendgrid-php/blob/master/lib/mail/Attachment.php#L78) method of the Attachment object.

See here for an example.

[martijnmelchers]

@thinkingserious

I could make this, but how would we detect if an attachment is base64 encoded?

[thinkingserious]

@martijnmelchers,

Thanks for offering to take this on!

I don’t know the best way to do this yet and I’m open to suggestions.

A quick search on the web reveals a bunch of solutions, but not foolproof.

In the end we should be sure to return an exception if the SendGrid API rejects the encoding.

With Best Regards,

Elmer