Nil as table index

If a **key** in a table is _nil_, the unpack will report an error. But it seems possible to create such an array in msgpack so maybe the Lua unpacker should just ignore it since it cannot be used this way.

Initially for me, the problems comes from the PHP msgpack extension that doesn't serialize sessions correctly and adds a nil indexed key in the array. I was trying to use redis as a session store to share sessions between Openresty (Lua) and PHP (I could do the same with Nodejs for example). The PHP unpack just ignores the nil indexed key/value. See the following issue for some details about the problem.
https://github.com/msgpack/msgpack-php/issues/50

There is the same issue in the Lua MessagePack implementation as described here:
https://github.com/fperrad/lua-MessagePack/issues/14