Add missing UnixDomainSocketConnection._buffer_cutoff
Add missing UnixDomainSocketConnection._buffer_cutoff

Without this using `unix_socket_path` will fail:

    AttributeError: 'UnixDomainSocketConnection' object has no attribute '_buffer_cutoff'

Fixes #1067

alisaifee/redis-py,5977862/redis-py,5977862/redis-py,andymccurdy/redis-py,alisaifee/redis-py,andymccurdy/redis-py,mozillazg/redis-py-doc,redis/redis-py,redis/redis-py,andymccurdy/redis-py,5977862/redis-py,mozillazg/redis-py-doc