Cannot supply Gson formatter for Bulk operations

[bdurand]

The code in Bulk creates a new Gson object for every Bulk action. This causes an issue where the JSON documents sent by Bulk are not the same as sent by individual Index actions where custom serialization is desired.

A getter and setter is provided for the Gson attribute, but the data is already serialized in the constructor so if you want to change the serialization, you have to do so twice.

Providing a mechanism to set the Gson object in the builder would be useful so it could use the same serializers across all actions.