// Non standard S3 HTTP response constants
const (
	XCache       = "X-Cache"
	XCacheLookup = "X-Cache-Lookup"
)

// Standard S3 HTTP request constants
const (
	IfModifiedSince   = "If-Modified-Since"
	IfUnmodifiedSince = "If-Unmodified-Since"
	IfMatch           = "If-Match"
	IfNoneMatch       = "If-None-Match"

	// Request tags used in GetObjectAttributes
	Checksum     = "Checksum"
	StorageClass = "StorageClass"

		}
	}

	// If-Unmodified-Since : Return the object only if it has not been modified since the specified
	// time, otherwise return a 412 (precondition failed).
	ifUnmodifiedSinceHeader := r.Header.Get(xhttp.IfUnmodifiedSince)
	if ifUnmodifiedSinceHeader != "" {
		if givenTime, err := amztime.ParseHeader(ifUnmodifiedSinceHeader); err == nil {
			if ifModifiedSince(objInfo.ModTime, givenTime) {