Description:
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I miss a serializer option to disable serializing.

If I only have string to serialize or would like to use an own serializer, a serializing by memcached isn't needed and makes stored data inconsistent.

This needs to throw an excaption if someone wonts to store a non-string on a serializer disabled memcached object.

Reproduce code:
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Example: usign same cache with PHP & Python

$pythonSerializer = new PythonPickleSerializer();
$memcached->setOption(Memcached::OPT_SERIALIZER, Memcached::SERIALIZER_NONE):
$memcached->set('id', $pythonSerializer->serialize($data));

Expected result:
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Reading data with python.

Actual result:
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Can't disable memcached build-in serializer and data will be serialized using one of the available serializer options.