Add test for overwrite arg

test/Function overwrite arg should overwrite existing globals.js

@@ -0,0 +1,12 @@
+import test from 'ava';
+import browserEnv from '../dist';
+
+test(t => {
+  t.is(typeof window, 'undefined');
+  let returnValue = browserEnv();
+  t.is(returnValue, window);
+  returnValue = browserEnv();
+  t.not(returnValue, window);
+  returnValue = browserEnv(['window'], true);
+  t.is(returnValue, window);
+});