Option to get the result as associative array

Problem/Motivation

In some cases is more convenient get the result as an associative array rather than objects.

Proposed resolution

Add an optional argument to the "CkanClientInterface:get()" method to request the return value as associative array.

Remaining tasks

Just add the argument and pass it in the json_decode() call.

User interface changes

None.

API changes

Adding an optional extra argument usually doesn't break the interface compatibility.

Data model changes

Not applicable.

Release notes snippet

TBD.