[WIP] Fix e.response is None when ChunkedEncodingError occured

Hi!
I found that it's impossible to read headers and response code when ChunkedEncodingError occured.

Here is demonstration code.
Server code:
```
#!/usr/bin/env python

import BaseHTTPServer
import SocketServer

class ChunkingHTTPServer(SocketServer.ThreadingMixIn,
                        BaseHTTPServer.HTTPServer):
    daemon_threads = True

class ChunkingRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
    ALWAYS_SEND_SOME = False
    ALLOW_GZIP = False
    protocol_version = 'HTTP/1.1'
    def do_GET(self):
        self.send_response(200)
        self.send_header('Transfer-Encoding', 'chunked')
        self.send_header('Content-type', 'text/plain')
        raise Exception()

if __name__ == '__main__':
    server = ChunkingHTTPServer(
        ('127.0.0.1', 8000), ChunkingRequestHandler)
    print 'Starting server, use <Ctrl-C> to stop'
    server.serve_forever()
```

Client code:
```
#!/usr/bin/env python

import requests

try:
    requests.get('http://127.0.0.1:8000')
except requests.exceptions.ChunkedEncodingError as e:
    print(e.response)
```

Before patch: None
After patch: <Response [200]>

I have problem with presenting this code for testing, just have no idea how to integrate it. Could someone help me with it, please.

Also I think that the same behavior is here:
 - https://github.com/requests/requests/blob/master/requests/models.py#L750
 - https://github.com/requests/requests/blob/master/requests/models.py#L752
But I don't know how to emulate these types of error



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> Merging [#4358](https://codecov.io/gh/requests/requests/pull/4358?src=pr&el=desc) into [master](https://codecov.io/gh/requests/requests/commit/81b63419cbd8a6f0c9c2b79083d25dc2260156e3?src=pr&el=desc) will **not change** coverage.
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```diff
@@           Coverage Diff           @@
##           master    #4358   +/-   ##
=======================================
  Coverage   88.28%   88.28%           
=======================================
  Files          18       18           
  Lines        2031     2031           
=======================================
  Hits         1793     1793           
  Misses        238      238
```


| [Impacted Files](https://codecov.io/gh/requests/requests/pull/4358?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [requests/models.py](https://codecov.io/gh/requests/requests/pull/4358?src=pr&el=tree#diff-cmVxdWVzdHMvbW9kZWxzLnB5) | `93.45% <0%> (ø)` | :arrow_up: |

------

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Are you sure this patch is necessary? If you set `stream=True` the error will be delayed until you read the body, allowing you to see the response header.

Hi @Lukasa thanks for your suggestion.
It's strange but when I use stream=True no exception raised at all and I'm not sure it's good.

As I noted, the exception should only happen when you read the body (e.g. when you call `response.content`, `response.iter_content`, `response.iter_lines` or similar).

Looking at this, Lukasa's explanation works fine. Using stream will allow access to the headers and if the server fails to send a complete response, we're not going to be able to display the response. You should have access to all other attributes on the object using stream though.

I'm going to close this out since this doesn't seem necessary. Thanks for the initiative though!