fix preconditions according to https://tools.ietf.org/id/draft-ietf-httpbis-p4-conditional-26.html#preconditions

weed/util/parse.go

@@ -61,3 +61,8 @@ func ParseHostPort(hostPort string) (filerServer string, filerPort int64, err er
 
 	return
 }
+
+func CanonicalizeETag(etag string) string {
+	canonicalETag := strings.TrimPrefix(etag, "\"")
+	return strings.TrimSuffix(canonicalETag, "\"")
+}