fix for api controllers without layouts

In spree-backend we include MultiDomainHelpers into the Api BaseController [here](https://github.com/bonobos/spree-backend/blob/dcc413c/app/controllers/api/base_controller_decorator.rb#L4)
That causes [this line](https://github.com/spree/spree-multi-domain/blob/64238a7/lib/spree_multi_domain/engine.rb#L25) to start returning true for Api controllers.
Then on the [following line](https://github.com/spree/spree-multi-domain/blob/64238a7/lib/spree_multi_domain/engine.rb#L26) the call to `layout.call.gsub...` blows up because "layout.call" is nil for Api controllers