Fix remoting metadata for "data" arguments

[bajtos]

Fix the definition of "data" argument to

{ type: 'object', model: modelName, ... }

That way strong-remoting passed the request body directly to the model
method (does not create a new model instance), but the swagger will
still provide correct schema for these arguments.

This fixes a bug where upsert in relation methods was adding default
property values to request payload.

backport #2702
connect to #1806