HEAD response with chunked Transfer-Encoding does not release the connection

Filing this to track the issue in urllib3 -- https://github.com/shazow/urllib3/issues/1234. Whenever urllib3 merges a fix, requests should update its requirements to avoid leaking connections.

The issue is that some servers -- in my case, Amazon S3 -- will set "Transfer-Encoding: chunked" even in response to a HEAD request. This results in urllib3 not returning the connection to the connection pool after requests calls `stream()` to get the content.

Here is a small script I wrote to confirm the problem (with a hacky reach into the pool).
```python

s = requests.Session()
req = requests.Request(method='HEAD', url='https://s3.amazonaws.com/other-bucket')
p = s.prepare_request(req)
resp = s.send(p, allow_redirects=False)
pool = s.adapters['https://'].poolmanager.pools._container.values()[0].pool
print pool.qsize()

s = requests.Session()
req = requests.Request(method='HEAD', url='https://google.com/other-bucket')
p = s.prepare_request(req)
resp = s.send(p, allow_redirects=False)
pool = s.adapters['https://'].poolmanager.pools._container.values()[0].pool
print pool.qsize()
```

In the case of google.com, the pool size is 10 upon completing the request, as the connection is put back. In the case of contacting Amazon, the pool size is 9. The default pool size is 10 and that's what we expect to get back.

Here's the debug information from the interaction, along with the requests version used:
```
2.18.1
DEBUG:urllib3.connectionpool:Starting new HTTPS connection (1): s3.amazonaws.com
send: 'HEAD /other-bucket HTTP/1.1\r\nHost: s3.amazonaws.com\r\nConnection: keep-alive\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nUser-Agent: python-requests/2.18.1\r\n\r\n'
reply: 'HTTP/1.1 301 Moved Permanently\r\n'
header: x-amz-bucket-region: ap-northeast-1
header: x-amz-request-id: A63B39B85D7E91DF
header: x-amz-id-2: BcZpRKNNvxorP7jG7SZE6RgcwFulAkuQDMBqw/q0C3DI6meKksIeZBYaeUD7pF3Xi3Mp5QjdObo=
header: Content-Type: application/xml
header: Transfer-Encoding: chunked
header: Date: Fri, 21 Jul 2017 08:01:49 GMT
header: Server: AmazonS3
DEBUG:urllib3.connectionpool:https://s3.amazonaws.com:443 "HEAD /other-bucket HTTP/1.1" 301 0
9
DEBUG:urllib3.connectionpool:Starting new HTTPS connection (1): google.com
send: 'HEAD /other-bucket HTTP/1.1\r\nHost: google.com\r\nConnection: keep-alive\r\nAccept-Encoding: gzip, deflate\r\nAccept: */*\r\nUser-Agent: python-requests/2.18.1\r\n\r\n'
reply: 'HTTP/1.1 404 Not Found\r\n'
header: Content-Type: text/html; charset=UTF-8
header: Referrer-Policy: no-referrer
header: Content-Length: 1573
header: Date: Fri, 21 Jul 2017 08:01:50 GMT
header: Alt-Svc: quic=":443"; ma=2592000; v="39,38,37,36,35"
DEBUG:urllib3.connectionpool:https://google.com:443 "HEAD /other-bucket HTTP/1.1" 404 0
10
```

As far as I can tell, this bug has been present ever since the `read_chunked()` method was introduced in urllib3.Response in 1.10.3.

As we don't vendor in urllib3 anymore, we don't need to open bug reports to track upstream issues. Thanks anyway!

Pursuant to @Lukasa 's comment, I'm closing this (as I suspect that was what he intended to do)