use static class fields instead of init logic (ember-learn#577)

Fix ember-learn#566

Author: efx

guides/release/models/customizing-adapters.md

@@ -209,22 +209,16 @@ Requests for `user-profile` would now target `/user_profile/1`.
 Some APIs require HTTP headers, e.g. to provide an API key. Arbitrary
 headers can be set as key/value pairs on the `JSONAPIAdapter`'s `headers`
 object and Ember Data will send them along with each ajax request.
-(Note that we set headers in `constructor()` because default property values
-should not be arrays or objects.)
 
 ```javascript {data-filename=app/adapters/application.js}
 import DS from 'ember-data';
 const { JSONAPIAdapter } = DS;
 
 export default class ApplicationAdapter extends JSONAPIAdapter {
-  constructor() {
-    super(...arguments);
-
-    this.headers = {
-      'API_KEY': 'secret key',
-      'ANOTHER_HEADER': 'Some header value'
-    };
-  }
+  headers = {
+    'API_KEY': 'secret key',
+    'ANOTHER_HEADER': 'Some header value'
+  };
 }
 ```