fix(isObservable): Fix throwing error when testing isObservable(null) (…

…#3688)

* fix(isObservable): Fix throwing error when testing isObservable(null)

* fix(isObservable): Use instanceOf instead

* fix(isObservable): Cleanup imports

spec/util/isObservable-spec.ts

@@ -23,4 +23,13 @@ describe('isObservable', () => {
 
     expect(isObservable(o)).to.be.false;
   });
+
+  it('should return false for null', () => {
+    expect(isObservable(null)).to.be.false;
+  });
+
+  it('should return false for a number', () => {
+    expect(isObservable(1)).to.be.false;
+  });
+
 });

src/internal/util/isObservable.ts

@@ -6,5 +6,5 @@ import { ObservableInput } from '../types';
  * @param obj the object to test
  */
 export function isObservable<T>(obj: any): obj is Observable<T> {
-  return obj && obj instanceof Observable || (typeof obj.lift === 'function' && typeof obj.subscribe === 'function');
+  return !!obj && (obj instanceof Observable || (typeof obj.lift === 'function' && typeof obj.subscribe === 'function'));
 }