Pass the base URL when expanding a templated `_links` href

[quietbits]

CallBuilder._requestFnForLink calls expandUriTemplate(link.href, opts) without its optional third argument, baseUrl. With no base, new URL(expanded, undefined) throws, so every templated link function on a record whose _links are relative fails with TypeError: Invalid URL. The non-templated branch on the next line passes this.url and resolves the same href correctly.

-        uri = new URL(expandUriTemplate(link.href, opts), this.url);
+        uri = new URL(expandUriTemplate(link.href, opts, this.url), this.url);

expandUriTemplate returns an absolute string, so the outer new URL() no longer needs its second argument.

Relative templated hrefs appear in the account-records fixture in test/unit/server/horizon/server.test.ts, but no test calls a link function on one.