[whatwg/fetch] why not add an option of json (#430)

currently, we write code as below if we need json response 
```js
fetch(myRequest)
.then(function(response) {
  return response.json();
})
.then(function(res) {
 // ...
});
```

wht not add an option of dataType optionally like jQuery
```
fetch(myRequest, {
  dataType: 'json'
})
.then(function(response) {
  return response.json();
})
.then(function(res) {
 // ...
});
```

@annevk  hope your reply

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