Pymemcache

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* Remove trailing space for commands that don't take arguments, such as
``stats``. This was a violation of the memcached protocol.

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* CAS operations will now raise ``MemcacheIllegalInputError`` when ``None`` is
given as the ``cas`` value.

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* Added IPv6 support for TCP socket connections. Note that IPv6 may be used in
preference to IPv4 when passing a domain name as the host if an IPv6 address
can be resolved for that domain.
* ``HashClient`` now supports UNIX sockets.

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* ``HashClient`` can now be imported from the top-level ``pymemcache`` package
(e.g. ``pymemcache.HashClient``).
* ``HashClient.get_many()`` now longer stores ``False`` for missing keys from
unavailable clients. Instead, the result won't contain the key at all.
* Added missing ``HashClient.close()`` and ``HashClient.quit()``.

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* ``PooledClient`` and ``HashClient`` now support custom ``Client`` classes

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* Improve ``MockMemcacheClient`` to behave even more like ``Client``