http.CanonicalHeaderKey(xhttp.AmzChecksumSHA256): true,
	http.CanonicalHeaderKey(xhttp.AmzChecksumMode):   true,
}

// Add policy conditionals.
const (
	policyCondEqual         = "eq"
	policyCondStartsWith    = "starts-with"
	policyCondContentLength = "content-length-range"
)

// toString - Safely convert interface to string without causing panic.
func toString(val interface{}) string {
	switch v := val.(type) {