HTTPAdapter set response.peercert to keep SSL certificate details before connection release

Hi guys,

To troubleshoot requests queries, I found useful to be able to access the connection SSL certificate details. Once the connection is closed, urllib3 call release_conn() and this information is not available afterwards, therefore I suggest to set a new attribute peercert in Response and set it in the HTTPAdapter.

Python 3.6

```python
(Pdb) type(response.raw._connection)
<class 'urllib3.connection.VerifiedHTTPSConnection'>
(Pdb) type(response.raw._connection.sock)
<class 'ssl.SSLSocket'>
````

```python
>>> resp = requests.get('https://s3.eu-west-1.amazonaws.com/')
>>> resp.peercert
{'subject': ((('commonName', 'aws.amazon.com'),),), 'issuer': ((('countryName', 'US'),), (('organizationName', 'Amazon'),), (('organizationalUnitName', 'Server CA 1B'),), (('commonName', 'Amazon'),)), 'version': 3, 'serialNumber': '0E9C9BE31387FA07A147D4406982B417', 'notBefore': 'Mar 28 00:00:00 2018 GMT', 'notAfter': 'Mar 28 12:00:00 2019 GMT', 'subjectAltName': (('DNS', 'aws.amazon.com'), ('DNS', 'www.aws.amazon.com'), ('DNS', 'aws-us-east-1.amazon.com'), ('DNS', 'aws-us-west-2.amazon.com'), ('DNS', 'amazonaws-china.com'), ('DNS', 'www.amazonaws-china.com')), 'OCSP': ('http://ocsp.sca1b.amazontrust.com',), 'caIssuers': ('http://crt.sca1b.amazontrust.com/sca1b.crt',), 'crlDistributionPoints': ('http://crl.sca1b.amazontrust.com/sca1b.crl',)}
```

With python 2.7 information are based on pyopenssl and less details are available though:

```
>>> resp = requests.get('https://s3.eu-west-1.amazonaws.com/')
>>> resp.peercert
{'subject': ((('commonName', u'aws.amazon.com'),),),
 'subjectAltName': [('DNS', 'aws.amazon.com'),
  ('DNS', 'www.aws.amazon.com'),
  ('DNS', 'aws-us-east-1.amazon.com'),
  ('DNS', 'aws-us-west-2.amazon.com'),
  ('DNS', 'amazonaws-china.com'),
  ('DNS', 'www.amazonaws-china.com')]}
```

```python
Pdb) type(response.raw._connection.sock)
<class 'urllib3.contrib.pyopenssl.WrappedSocket'>
(Pdb) type(response.raw._connection)
<class 'urllib3.connection.VerifiedHTTPSConnection'>
```
I also found people interested by this on [stackoverflow](https://stackoverflow.com/questions/16903528/how-to-get-response-ssl-certificate-from-requests-in-python)

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> The diff coverage is `100%`.

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```diff
@@           Coverage Diff            @@
##           master   #4736     +/-   ##
========================================
+ Coverage   66.79%   66.9%   +0.1%     
========================================
  Files          15      15             
  Lines        1563    1568      +5     
========================================
+ Hits         1044    1049      +5     
  Misses        519     519
```


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|---|---|---|
| [requests/models.py](https://codecov.io/gh/requests/requests/pull/4736/diff?src=pr&el=tree#diff-cmVxdWVzdHMvbW9kZWxzLnB5) | `77.62% <100%> (+0.05%)` | :arrow_up: |
| [requests/adapters.py](https://codecov.io/gh/requests/requests/pull/4736/diff?src=pr&el=tree#diff-cmVxdWVzdHMvYWRhcHRlcnMucHk=) | `70.72% <100%> (+0.53%)` | :arrow_up: |

------

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Hi there, 

Thanks for taking the time to put together this PR. Unfortunately, Requests is not accepting new features at the moment. Given that this is a customization to our existing HTTPAdapter, you should be able to make your own adapter that sub-classes it and add this to it.

Cheers