Parameterized TOSCA template doesn't work

I am not able to pass:

:attributes => {:param_values => params}})

where params is:
params = JSON.dump({'image_name' => 'cirros', 'flavor_name' => 'm1.small'})

Here is the full example:
https://gist.github.com/Ladas/b1c186b9bbefe0122604ee27fcfb62cb

So users need to dump the param values into JSON, so it saves to DB without issues and we need to parse it from JSON and encode, so it can be correctly used in Heat template