Allow binary content to be returned from PHP

[joelbarnard]

To return Binary content through the ALB, it needs to be Base64 Encoded and the isBase64Encoded flag needs to be set to true.

This change Base64 encodes all responses, and sets the isBase64Encoded flag to true, so that both plaintext and binary responses work.

[txase]

This is helpful, though it has an issue in that it will base64 encode a lot of things that don't need it. Can we validate whether the body is invalid utf-8 first, and only then encode it?