Option to set server name in HTTPs request

Summary.
Server name is being read from the request URI resulting in SSL handshake failure when making request to IP address. Setting host_header does not help as the value of ssl.handshake.extensions_server_name is still being set to the IP address.

## Expected Result
Need an option similar to openssl s_client  -servername
https://wiki.openssl.org/index.php/Manual:S_client(1)
What you expected.

## Actual Result

What happened instead.
ssl.handshake.extensions_server_name is being set to the URI

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 560, in urlopen
    body=body, headers=headers)
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 346, in _make_request
    self._validate_conn(conn)
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 787, in _validate_conn
    conn.connect()
  File "/usr/lib/python3/dist-packages/urllib3/connection.py", line 252, in connect
    ssl_version=resolved_ssl_version)
  File "/usr/lib/python3/dist-packages/urllib3/util/ssl_.py", line 305, in ssl_wrap_socket
    return context.wrap_socket(sock, server_hostname=server_hostname)
  File "/usr/lib/python3.5/ssl.py", line 377, in wrap_socket
    _context=self)
  File "/usr/lib/python3.5/ssl.py", line 752, in __init__
    self.do_handshake()
  File "/usr/lib/python3.5/ssl.py", line 988, in do_handshake
    self._sslobj.do_handshake()
  File "/usr/lib/python3.5/ssl.py", line 633, in do_handshake
    self._sslobj.do_handshake()
ssl.SSLError: [SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:645)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 376, in send
    timeout=timeout
  File "/usr/lib/python3/dist-packages/urllib3/connectionpool.py", line 589, in urlopen
    raise SSLError(e)
requests.packages.urllib3.exceptions.SSLError: [SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:645)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "python_requests_sni.py", line 4, in <module>
    r = requests.get(request_url,headers=headers,verify=False)
  File "/usr/lib/python3/dist-packages/requests/api.py", line 67, in get
    return request('get', url, params=params, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/api.py", line 53, in request
    return session.request(method=method, url=url, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 468, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/lib/python3/dist-packages/requests/sessions.py", line 576, in send
    r = adapter.send(request, **kwargs)
  File "/usr/lib/python3/dist-packages/requests/adapters.py", line 447, in send
    raise SSLError(e, request=request)
requests.exceptions.SSLError: [SSL: SSLV3_ALERT_HANDSHAKE_FAILURE] sslv3 alert handshake failure (_ssl.c:645)

## Reproduction Steps
import requests
headers = {'host': 'host_header'}
request_url = "https://1.1.1.1/index.html"
r = requests.get(request_url,headers=headers,verify=False)
print(r.content)

```python
import requests

```

## System Information

    $ python -m requests.help

```
<paste here>
```

This command is only available on Requests v2.16.4 and greater. Otherwise,
please provide some basic information about your system (Python version,
operating system, &c).

Operating system -
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.1 LTS
Release:	16.04
Codename:	xenial

Python version -
Python 3.5.2

That's provided by this [transport adapter](http://toolbelt.readthedocs.io/en/latest/adapters.html#hostheaderssladapter).

Cheers!