Return `head :no_content` instead of `:ok`

`head :ok` returns a blank space (`" "`) in the body of the response, which can
cause jQuery to choke if it tries to parse the body as JSON.

This change uses `head :no_content` instead, which is how Rails' `ActionController::Responder` class responds to `:put` or `:delete` requests in the `api_behavior` method.

Fixes #220.