Fix str encode for request with proxy (Py3)

to fix this error:

```python
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    conn.send_email('foo@bar.com', 'Foo!', 'Bar', ['bar@foo.com])
  File "C:\Python34\lib\site-packages\boto\ses\connection.py", line 276, in send_email
    return self._make_request('SendEmail', params)
  File "C:\Python34\lib\site-packages\boto\ses\connection.py", line 102, in _make_request
    data=urllib.parse.urlencode(params)
  File "C:\Python34\lib\site-packages\boto\connection.py", line 1068, in make_request
    retry_handler=retry_handler)
  File "C:\Python34\lib\site-packages\boto\connection.py", line 913, in _mexe
    self.is_secure)
  File "C:\Python34\lib\site-packages\boto\connection.py", line 705, in get_http_connection
    return self.new_http_connection(host, port, is_secure)
  File "C:\Python34\lib\site-packages\boto\connection.py", line 747, in new_http_connection
    connection = self.proxy_ssl(host, is_secure and 443 or 80)
  File "C:\Python34\lib\site-packages\boto\connection.py", line 796, in proxy_ssl
    sock.sendall("CONNECT %s HTTP/1.0\r\n" % host)
TypeError: 'str' does not support the buffer interface
```