Prevent colliding internal cache ids based on adapter

[weierophinney]

Resolves #41
json_encode($this->getAdapter()) would always return an empty object for \Zend\Paginator\Adapter\DbSelect adapters (i.e. {}). By performing print_r on the adapter we can get the current state which, in the case of DbSelect, includes an sql object containing table name, join details, where clause, etc.
This change will prevent collisions between adapters that do not have any public properties and/or values.

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Originally posted by @richard-parnaby-king at zendframework/zend-paginator#42

[froschdesign]

Duplicate of #3