# File lib/aws/core/signers/version_3.rb, line 35
        def sign_request req
          req.headers["x-amz-date"] ||= (req.headers["date"] ||= Time.now.httpdate)
          req.headers["host"] ||= req.host
          req.headers["x-amz-security-token"] = credentials.session_token if
            credentials.session_token
          req.headers["x-amzn-authorization"] =
            "AWS3 "+
            "AWSAccessKeyId=#{credentials.access_key_id},"+
            "Algorithm=HmacSHA256,"+
            "SignedHeaders=#{headers_to_sign(req).join(';')},"+
            "Signature=#{signature(req)}"
        end