tested by sourcing proxmox-acme and the bundled dns_ali.sh, and building
the request URL of _ali_rest for a fixed query with a mocked _get. with
this patch the signature matches the one of a reference implementation
in Python, which encodes with upper-case hex as Aliyun requires, without
it it does not. the default lower-case output of _url_encode is
unchanged. 'make test' in src passes.