Enable tuple responses with headers without status

Hi,

In my flask apps I've overridden the behavior of Flask.make_response to enable tuple responses with headers but skipping the status in the case of the default status (200). I know that this in practice only means a less coma and status, but thinks this enable a more beautiful way to return tuple responses with the default status code (or status directly passed into the Response constructor) but contains other headers.

So instead of today
```python
@app.route("/")
def index():
    return "hi", 200, {"X-Foo": "Bar"} 
```

This PR enable us to write
```python
@app.route("/")
def index():
    return "hi", {"X-Foo": "Bar"} 
```