Fix DuckDNS double-suffixing a subdomain that already includes .duckdns.org (#128)

Reported error when adding a DuckDNS provider with
subdomains="nl-theta42.duckdns.org" (the full name, as DuckDNS's own
site displays it):

  {"name": "EntryNotFound", "message": "Domain:duckdns.org does not exists"}

listDomains() blindly appended ".duckdns.org" to whatever was entered,
turning "nl-theta42.duckdns.org" into
"nl-theta42.duckdns.org.duckdns.org". tld-extract doesn't know
duckdns.org is a shared suffix, so it parsed that malformed string
down to domain "duckdns.org" — surfacing as a confusing EntryNotFound
two layers away from the actual cause (Domain.create's internal
lookup).

Add __normalizeLabel() to strip a trailing ".duckdns.org" (and
lowercase) before use, so both "myhost" and "myhost.duckdns.org" work
identically. Also make __label()'s existing suffix-strip
case-insensitive to match.

nodejs/test/integration/dns_provider.test.js

@@ -200,6 +200,25 @@ describe('DNS Provider Contract Compliance', () => {
 			const instance = new DuckDns({token: 'mock-token', subdomains: 'mockhost'});
 			assert.strictEqual(instance.__label({domain: 'mockhost.duckdns.org'}), 'mockhost');
 		});
+
+		test('__normalizeLabel accepts both the bare label and the full duckdns.org name', () => {
+			const instance = new DuckDns({token: 'mock-token', subdomains: 'mockhost'});
+			assert.strictEqual(instance.__normalizeLabel('mockhost'), 'mockhost');
+			assert.strictEqual(instance.__normalizeLabel('mockhost.duckdns.org'), 'mockhost');
+			assert.strictEqual(instance.__normalizeLabel('MockHost.DuckDNS.org'), 'mockhost');
+		});
+
+		test('listDomains does not double-suffix a subdomains value that already includes .duckdns.org', async () => {
+			const instance = new DuckDns({token: 'mock-token', subdomains: 'nl-theta42.duckdns.org,other'});
+			// Stub out the network call — this test is only about what domain
+			// name(s) listDomains() builds from `subdomains`, not the live API.
+			instance.update = async () => {};
+			const domains = await instance.listDomains();
+			assert.deepStrictEqual(domains, [
+				{domain: 'nl-theta42.duckdns.org'},
+				{domain: 'other.duckdns.org'},
+			]);
+		});
 	});
 });