Return the sha256 digest of the content in the
        header format the Merchant API expects.

def _get_sha256_digest(self, content):
        """Return the sha256 digest of the content in the
        header format the Merchant API expects.
        """
        content_sha256 = base64.b64encode(SHA256.new(content).digest())
        return 'SHA256=' + content_sha256