Improved Open Graph support

The `api` method currently encode in Json arrays passed as parameters. This produces errors when trying to send user generated photos :

> (#3503) \*Json string\* is an invalid value for property "image:url" with type "URL" 

or when tagging people :

> (#100) param tags must be an array.

As far as I know only FQL queries need to be encoded in Json. So with this pull request arrays are only encoded in Json when this is a FQL query.