fix: move to a singleton for tracking Helmet instances

This resolves an issue where multiple HelmetProviders on the same
page don't know about each other.

Author: blittle

jest.setup.js

@@ -2,6 +2,7 @@ import 'raf/polyfill';
 import { configure } from 'enzyme';
 import Adapter from 'enzyme-adapter-react-16';
 import ReactDOM from 'react-dom';
+import { clearInstances } from './src/HelmetData';
 
 configure({ adapter: new Adapter() });
 
@@ -40,4 +41,5 @@ beforeEach(() => {
 
 afterEach(() => {
   ReactDOM.unmountComponentAtNode(mount);
+  clearInstances();
 });

src/HelmetData.js

@@ -1,31 +1,31 @@
 import mapStateOnServer from './server';
 
-export default class HelmetData {
-  instances = [];
+const instances = [];
+
+export function clearInstances() {
+  instances.length = 0;
+}
 
+export default class HelmetData {
   value = {
     setHelmet: serverState => {
       this.context.helmet = serverState;
     },
     helmetInstances: {
-      get: () => this.instances,
+      get: () => instances,
       add: instance => {
-        this.instances.push(instance);
+        instances.push(instance);
       },
       remove: instance => {
-        const index = this.instances.indexOf(instance);
-        this.instances.splice(index, 1);
+        const index = instances.indexOf(instance);
+        instances.splice(index, 1);
       },
     },
   };
 
-  constructor(context, instances) {
+  constructor(context) {
     this.context = context;
 
-    if (instances) {
-      this.instances = instances;
-    }
-
     if (!HelmetData.canUseDOM) {
       context.helmet = mapStateOnServer({
         baseTag: [],