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

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

@@ -1,10 +1,13 @@
 import { Observable } from '../Observable';
 import { ObservableInput } from '../types';
+import { isObject  } from './isObject';
+import { isFunction } from './isFunction';
 
 /**
  * Tests to see if the object is an RxJS {@link Observable}
  * @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 ||
+    (isObject(obj) || isFunction(obj)) ? (isFunction(obj.lift) && isFunction(obj.subscribe)) : false;
 }