A little fix to separate the get request with auth header and without au...

...th header

@longbai 

Do this fix to support code like this:

```python

from qiniu import http

url = "http://api.qiniu.com/status/get/prefop"
params = {"id": "54898c247823de40683b32cc"}
retData, respInfo = http._get(url, params)
print(retData)
print(respInfo)
```
 auth = RequestsAuth(auth) if auth is not None else None
其他的改动都不要