read_json from url, Accept Header?
Today I tried to read json data from an url that checks the Accept-Header for 'application/json',
and only delivers json if this tag is higher ranked than 'text/html'.

See: [Flask: Handling Accept Headers](http://flask.pocoo.org/snippets/45/)
It seems, that the pandas request in '_url_open' has no such header at all.

This works:

``` python
import requests
import pandas
url = "http://localhost:5000/foo"
r = requests.get(url, headers={'Accept': 'application/json'})
data = pandas.DataFrame(r.json())
print(data.head())
```

This does not, and I checked the header in the flask webapp of the received request, it is empty:

``` python
import pandas
url = "http://localhost:5000/foo"
data = pandas.read_json(url)
print(data.head())
```

It would make sense if the `read_json` had `application/json` in its Accept-Header, correct?
