Dict with `NaN` values been serialised to malformed JSON

Using Python 3.6

Below is the most basic example of the issue

**app.py**

```
@app.route('/mytest', methods=['GET'])
def mytest():
data = {
"number": float('NaN')
}
return data
```

This will return in this **invalid** JSON

```
{
"number": NaN
}
```

This is the same JSON which is generated by

```
data = {
"number": float('NaN')
}
json.dumps(data)
```

Start with the app.py reproduction and compare the endpoint response with Python's json.dumps(data) behavior for float('NaN'). Trace the response serialization entry point and its tests; done means the reported NaN case no longer produces malformed JSON, with the expected handling captured in a test.

Written by the indexing model from the issue text.