Add test to check existing falsey globals don't get overwritten

test/Existing falsey node globals don't get overwritten.js

@@ -0,0 +1,9 @@
+import test from 'ava';
+
+// We have to require() here as imports have to be top level so we can't set
+// globals first
+test(t => {
+  global.document = false;
+  require('../dist')();
+  t.is(document, false);
+});