Mocking fetch.md

successful Fetch

    window.fetch = (url) =>{  
      return Promise.resolve({  
        json:()=> Promise.resolve({  
          slip:{advice:"Don't eat yellow snow"}  
        })  
      })  
    };  

Rejected fetch

```
window.fetch = (url) =>{  
  return Promise.reject(  'Woops'  )  
};  

```