fixed assignment default_error_formatter

I created json api with settings:
```ruby
class Application < Grape::API
  ...
  content_type :json, "application/json"
  default_error_formatter :json
  error_formatter :json, ::Userbase::Errors::JsonFormatter
  ...
end
```
When I make request '/something' without '.json' I get rescue
```ruby
NoMethodError:
 undefined method `call' for :json:Symbol
# ./lib/grape/middleware/error.rb:75:in `format_message'
# ./lib/grape/middleware/error.rb:63:in `error_response'
# ./lib/grape/middleware/error.rb:27:in `call!'
# ./lib/grape/middleware/base.rb:18:in `call'
# ./lib/grape/endpoint.rb:155:in `call!'
# ./lib/grape/endpoint.rb:145:in `call'
# ./lib/grape/api.rb:493:in `call'
# ./lib/grape/api.rb:42:in `call!'
# ./lib/grape/api.rb:38:in `call'
```
Because in method ```default_error_formatter``` assigned name formatter instead of being looking for via formatter_for.