Run ActiveRecord's quoting machinery

Thanks to this Date, DateTime and Time objects are always converted to the format acceptable by BigQuery.
This change also disables the actual quoting of strings - this is not needed as the JSON serializer does this.

@michelson, I was also thinking about a small optimization - we can skip running Base::quote() in line #89 for values that are strings (and maybe other types too). The code is clearer now and harmonised with ActiveRecord's flow, but does unnecessary things.