Change the way I checked for Headers

Apparent `windows` is undefined in Node v12.11.1.
Checked the Headers object directly instead.  Less complications involved.

Fix #5

src/createRequestOptions.js

@@ -5,7 +5,10 @@ const setHeaders = ({
   method = 'get',
   auth
 } = {}) => {
-  if (!window.Headers) require('isomorphic-fetch')
+  if (typeof Headers === 'undefined') {
+    require('isomorphic-fetch')
+  }
+
   const h = new Headers(headers)
 
   // Sets content type to 'application/json' for POST, PUT, PATCH, DELETE requests

test/index.spec.js

@@ -1,5 +1,4 @@
 /* globals beforeEach afterEach describe expect it */
-require('isomorphic-fetch')
 const portastic = require('portastic')
 const zlFetch = require('../src/index')
 const app = require('./helpers/createServer')