Rewrite and better explain config param filtering

Rewrite the config param filtering to make it more obvious what is going
on and extract a couple of helper functions to make the test more
readable.

src/annotator/host.js

@@ -31,19 +31,19 @@ export default class Host extends Guest {
       }
     }
 
-    // Make a copy of all config settings except `config.sidebarAppUrl`, the app base URL,
-    // and `config.pluginClasses`
-    const configParam =
-      'config=' +
-      encodeURIComponent(
-        JSON.stringify(
-          Object.assign({}, config, {
-            sidebarAppUrl: undefined,
-            pluginClasses: undefined,
-          })
-        )
+    // Make a copy of the config for use by the sidebar app with several
+    // annotator-only properties removed. nb. We don't currently strip all the
+    // annotator-only properties here. That's OK because validation / filtering
+    // happens in the sidebar app itself. It just results in unnecessary content
+    // in the sidebar iframe's URL string.
+    const sidebarConfig = { ...config };
+    ['sidebarAppUrl', 'pluginClasses'].forEach(
+      key => delete sidebarConfig[key]
     );
 
+    const configParam =
+      'config=' + encodeURIComponent(JSON.stringify(sidebarConfig));
+
     const sidebarAppSrc = config.sidebarAppUrl + '#' + configParam;
 
     // Create the iframe

src/annotator/test/host-test.js

@@ -97,18 +97,23 @@ describe('Host', () => {
       host.publish('panelReady');
     });
 
+    function getConfigString(host) {
+      return host.frame[0].children[0].src;
+    }
+
+    function configFragment(config) {
+      return '#config=' + encodeURIComponent(JSON.stringify(config));
+    }
+
     it('passes config to the sidebar iframe', () => {
       const appURL = new URL(
         '/base/annotator/test/empty.html',
         window.location.href
       );
       const host = createHost({ annotations: '1234' });
-      const configStr = encodeURIComponent(
-        JSON.stringify({ annotations: '1234' })
-      );
       assert.equal(
-        host.frame[0].children[0].src,
-        appURL + '#config=' + configStr
+        getConfigString(host),
+        appURL + configFragment({ annotations: '1234' })
       );
     });