Fix handling of permanent redirect detection

For some reason, the code assumed that the fetch API would not follow permanent
redirects.

The fetch API follows redirects by default and that's a good thing as requests
on the `/specifications/[shortname]/versions/latest` API endpoint typically
return a 302 to the actual version that is the current latest version.

This update rather relies on the `redirected` flag to detect redirects and
checks the final URL to see whether the W3C API knows the spec under a
different shortname.

src/fetch-info.js

@@ -99,17 +99,20 @@ async function fetchInfoFromW3CApi(specs, options) {
     if (res.status === 404) {
       return;
     }
-    if (res.status === 301) {
-      const rawLocation = res.headers.get('location');
-      const location = rawLocation.startsWith('/specifications/') ?
-        rawLocation.substring('/specifications/'.length) :
-        rawLocation.location;
-      throw new Error(`W3C API redirected to "${location}" ` +
-        `for "${spec.shortname}" (${spec.url}), update the shortname!`);
-    }
     if (res.status !== 200) {
       throw new Error(`W3C API returned an error, status code is ${res.status}, url was ${url}`);
     }
+
+    // Has the shortname changed from a W3C perspective?
+    if (res.redirected) {
+      const match = res.url.match(/\/specifications\/([^\/]+)\//);
+      const w3cShortname = match ? match[1] : '';
+      if (w3cShortname !== spec.shortname) {
+        throw new Error(`W3C API redirects "${spec.shortname}" to ` +
+          `"${w3cShortname}", update the shortname!`);
+      }
+    }
+
     try {
       const body = await res.json();
       return body;

test/fetch-info.js

@@ -255,7 +255,7 @@ describe("fetch-info module", function () {
     });
   });
 
-    describe("fetch from W3C API", () => {
+  describe("fetch from W3C API", () => {
     it("works on a TR spec", async () => {
       const spec = getW3CSpec("hr-time-2", "hr-time");
       const info = await fetchInfo([spec]);
@@ -313,6 +313,16 @@ describe("fetch-info module", function () {
       assert.equal(info.__series["CSS"].title, "Cascading Style Sheets");
       assert.equal(info.__series["css"].title, "CSS Snapshot");
     });
+
+    it("detects redirects", async () => {
+      const spec = {
+        url: "https://www.w3.org/TR/webaudio/",
+        shortname: "webaudio"
+      };
+      await assert.rejects(
+        fetchInfo([spec]),
+        /^Error: W3C API redirects "webaudio" to "webaudio-.*"/);
+    });
   });
 
   describe("fetch from Specref", () => {