Fix browser tests

Author: monkbroc

fs.js

@@ -0,0 +1,2 @@
+// In Node, exports the fs module. In the browser, exports undefined due to "./fs": false entry in package.json
+module.exports = require('fs');

package.json

@@ -83,7 +83,7 @@
     "stream-http": "^3.2.0"
   },
   "browser": {
-    "fs": false,
+    "./fs": false,
     "http": "stream-http",
     "https": "stream-http"
   },

src/Agent.js

@@ -20,7 +20,7 @@
 import fetch from 'node-fetch';
 import FormData from 'form-data';
 import qs from 'qs';
-import fs from 'fs';
+import fs from '../fs';
 import packageJson from '../package.json';
 
 /**