simpleleveldb does not correctly state Content-Type

``` python
http = httpclient.HTTPClient()
url = "http://localhost:13731/get"
params = urllib.urlencode({'key':'f00'})
url += '?' + params
resp = http.fetch(url)
print resp.headers.get('Content-Type', "No content type!") # prints: text/html; charset=ISO-8859-1
```

Should be `application/json` or `text/javascript`
