Changed the direct call of `tr` to cast to lower case to the function

`_lower_case` which is already provided.

dnsapi/dns_dynv6.sh

@@ -16,7 +16,7 @@ dynv6_api="https://dynv6.com/api/v2"
 # Please Read this guide first: https://github.com/Neilpang/acme.sh/wiki/DNS-API-Dev-Guide
 #Usage: dns_dynv6_add  _acme-challenge.www.domain.com  "XKrxpRBosdIKFzxW_CT3KLZNf6q0HG9i01zxXp5CPBs"
 dns_dynv6_add() {
-  fulldomain=$(echo "$1" | tr '[:upper:]' '[:lower:]')
+  fulldomain="$(echo "$1" | _lower_case)"
   txtvalue="$2"
   _info "Using dynv6 api"
   _debug fulldomain "$fulldomain"
@@ -50,7 +50,7 @@ dns_dynv6_add() {
 #Usage: fulldomain txtvalue
 #Remove the txt record after validation.
 dns_dynv6_rm() {
-  fulldomain=$(echo "$1" | tr '[:upper:]' '[:lower:]')
+  fulldomain="$(echo "$1" | _lower_case)"
   txtvalue="$2"
   _info "Using dynv6 API"
   _debug fulldomain "$fulldomain"