VCRHTTPResponse should have a release_conn() method

[Antiochian]

Similar slightly to #117

The urllib3 library's response class has the method release_conn(), intended for use with connection pools:
https://urllib3.readthedocs.io/en/latest/reference/urllib3.response.html#urllib3.response.BaseHTTPResponse.release_conn

When I try to use VCR with my code that uses urllib3 connection pools, I get the following exception:

    def authenticate_user(self, response, **kwargs):
        """Handles user authentication with gssapi/kerberos"""
    
        host = urlparse(response.url).hostname
    
        try:
            auth_header = self.generate_request_header(response, host)
        except KerberosExchangeError:
            # GSS Failure, return existing response
            return response
    
        log.debug("authenticate_user(): Authorization header: {0}".format(
            auth_header))
        response.request.headers['Authorization'] = auth_header
    
        # Consume the content so we can reuse the connection for the next
        # request.
        response.content
>       response.raw.release_conn()
E       AttributeError: 'VCRHTTPResponse' object has no attribute 'release_conn'. Did you mean: '_close_conn'?

I can see that in urllib3_stubs.py we set urllib3 classes as base classes for VCRRequestsHTTPConnection and VCRRequestsHTTPSConnection.

Is the solution to add a similar snippet in urllib3_stubs.py to set the base class of `VCRHTTPResponse?