Default JSON error handler to reject message

[brerx]

I would prefer this as a default. All the code where codecov is enabled currently has to look like this:

  use Ears::Middlewares::JSON,
      # :nocov:
      on_error: Proc.new { :reject }
  # :nocov:

this is ugly.

Any objections?

[Fruetel]

Seems like a good idea.