Support passing a specific connection class to StrictRedisCluster

[ilr39]

This cannot be done by using kwargs because kwargs are later passed on to the super init(which is StrictRedis) and it doesn't support arbitrary kwargs, so it fails when trying to pass connection_class as named argument to StrictRedis constructor.

StrictRedis constructor:
def __init__(self, host='localhost', port=6379, db=0, password=None, socket_timeout=None, socket_connect_timeout=None, socket_keepalive=None, socket_keepalive_options=None, connection_pool=None, unix_socket_path=None, encoding='utf-8', encoding_errors='strict', charset=None, errors=None, decode_responses=False, retry_on_timeout=False, ssl=False, ssl_keyfile=None, ssl_certfile=None, ssl_cert_reqs=None, ssl_ca_certs=None, max_connections=None):

This doesn't break any interface.
I need this to use SSL connection class with my cluster connection.

[coveralls]

Coverage increased (+0.05%) to 86.239% when pulling 8e483b9 on ilr39:support-passing-connection-class into 8e547a2 on Grokzen:unstable.

[Grokzen]

@ilr39 Great find and thanks for the contribution