Fix DuckDNS domains field colliding with DnsProvider's own relation (#127)

Reported error when adding a DuckDNS provider:

  TypeError: this.domains.map is not a function
    at Proxy.updateDomains (models/dns_provider.js:185:37)

DnsProvider.__intraModel merges `{...DnsProvider._keyMap,
...Provider._keyMap}`, so a provider-defined field with the same name
as one of DnsProvider's own (created_by, updated_by, name,
dnsProvider, domains, id) silently overwrites it. DuckDNS defined a
`domains` field (the operator-supplied comma-separated subdomain
list), which replaced DnsProvider's `domains` relation (rel: 'many' to
Domain, populated by updateDomains()) — so `this.domains` stopped
being the array relation and became DuckDNS's raw string instead.

Rename the field to `subdomains` throughout (model, docs, tests). Add
a comment on __intraModel documenting the collision risk for future
providers, and a regression test asserting no registered provider's
_keyMap redefines one of DnsProvider's reserved field names.

docs/api.md

@@ -727,11 +727,11 @@ curl -H "Content-Type: application/json" \
 curl -H "Content-Type: application/json" \
   -H "auth-token: your-token-here" \
   -X POST \
-  -d '{"name": "My DuckDNS", "dnsProvider": "DuckDns", "token": "your-duckdns-token", "domains": "myhost,myhost2"}' \
+  -d '{"name": "My DuckDNS", "dnsProvider": "DuckDns", "token": "your-duckdns-token", "subdomains": "myhost,myhost2"}' \
   https://proxy-host.com/api/dns
 ```
 
-`domains` is a comma-separated list of the subdomains you've registered at
+`subdomains` is a comma-separated list of the subdomains you've registered at
 [duckdns.org](https://www.duckdns.org) (e.g. `myhost` for
 `myhost.duckdns.org`), since DuckDNS has no API to list them for you.
 DuckDNS only supports one A/AAAA record and one TXT record per domain (no