Fix User-Agent request header (#40)

I thought the previous implementation would cause the undefined
User-Agent to be omitted from the request headers, but it was
actually being included (triggering CORS). I'm dumb. Added a test to
prevent this in the future.

src/internal/haveibeenpwned/axiosInstance.js

@@ -1,10 +1,6 @@
 import Axios from 'axios';
 import { name, version } from '../../../package.json';
 
-// Add a custom User-Agent header when running outside the browser
-const customUserAgent =
-  typeof navigator === 'undefined' ? `${name} ${version}` : undefined;
-
 /**
  * An Axios instance used for API queries. Not meant for general use.
  *
@@ -14,6 +10,9 @@ export default Axios.create({
   baseURL: 'https://haveibeenpwned.com/api',
   headers: {
     Accept: 'application/vnd.haveibeenpwned.v2+json',
-    'User-Agent': customUserAgent,
+    // Add a custom User-Agent header when running outside the browser
+    ...(typeof navigator === 'undefined' && {
+      'User-Agent': `${name} ${version}`,
+    }),
   },
 });

src/internal/haveibeenpwned/axiosInstance.test.js

@@ -7,6 +7,9 @@ describe('internal (haveibeenpwned): axiosInstance', () => {
     // Node
     global.navigator = undefined;
     const axiosInstanceNode = require.requireActual('./axiosInstance').default;
+    expect(Object.keys(axiosInstanceNode.defaults.headers)).toContain(
+      'User-Agent',
+    );
     expect(axiosInstanceNode.defaults.headers['User-Agent']).toBeTruthy();
 
     jest.resetModules();
@@ -15,7 +18,9 @@ describe('internal (haveibeenpwned): axiosInstance', () => {
     global.navigator = {};
     const axiosInstanceBrowser = require.requireActual('./axiosInstance')
       .default;
-    expect(axiosInstanceBrowser.defaults.headers['User-Agent']).toBeUndefined();
+    expect(Object.keys(axiosInstanceBrowser.defaults.headers)).not.toContain(
+      'User-Agent',
+    );
 
     global.navigator = originalNavigator;
   });