Adjust the way ParsePromise.is detects thenables

Author: andrewimm

src/ParsePromise.js

@@ -283,7 +283,7 @@ export default class ParsePromise {
    */
   static is(promise) {
     return (
-      typeof promise !== 'undefined' &&
+      promise != null &&
       typeof promise.then === 'function'
     );
   }

src/__tests__/ParsePromise-test.js

@@ -422,4 +422,13 @@ describe('Promise', () => {
       expect(error).toBe('hello');
     });
   });
+
+  it('can check if an object is a thenable promise', () => {
+    expect(ParsePromise.is(null)).toBe(false);
+    expect(ParsePromise.is(void(0))).toBe(false);
+    expect(ParsePromise.is('a string')).toBe(false);
+    expect(ParsePromise.is({})).toBe(false);
+    expect(ParsePromise.is(ParsePromise.as())).toBe(true);
+    expect(ParsePromise.is(ParsePromise.error())).toBe(true);
+  })
 });