Fix JSON formatting of resource type

[theravel]

I think it is well-known that json_encode does not handle variables of resource type properly. Easiest example is:

php -r "var_dump(json_encode(['test' => curl_init()]));" 
// returns bool(false)

JsonFormatter of Monolog is affected as well, so it returns empty line if there is a resource in "context".

This pull request leverages already existing code from NormalizerFormatter to convert resources to sprintf('[resource(%s)]', get_resource_type($data)).

[Seldaek]

Thanks, merged in 1.x & master