Add missing spec for validation exception

Author: stjhimy

spec/grape/exceptions/validation_spec.rb

@@ -1,6 +1,10 @@
 require 'spec_helper'
 
 describe Grape::Exceptions::Validation do
+  it 'fails when params are missing' do
+    expect { Grape::Exceptions::Validation.new(message_key: 'presence') }.to raise_error(RuntimeError, 'Params are missing:')
+  end
+
   it 'store message_key' do
     expect(Grape::Exceptions::Validation.new(params: ['id'], message_key: 'presence').message_key).to eq('presence')
   end