added spec for expected headers when error raised

When we set headers using `before`, `after` or just before `error!` raised, we expect them to transferred with error response together.

* First spec will be `success` due to there is no error.
* Second test will fail at the moment because when an error raised, all headers will be ignored. They shouldn't be.

Author: gencer

spec/grape/headers_on_error_spec.rb

@@ -1,7 +1,7 @@
 require 'spec_helper'
 
-describe Grape::API do 
- let(:error_header) do
+describe Grape::API do
+  let(:error_header) do
     Class.new(Grape::API) do
       before do
         header 'X-Grape-Before-Header', '1'
@@ -14,7 +14,7 @@
       end
       get '/error' do
         header 'X-Grape-Returns-Error', '1'
-        error!({ success: false })
+        error!(success: false)
       end
     end
   end
@@ -37,7 +37,7 @@ def app
     expect(last_response.headers['X-Grape-Before-Header']).to eq('1')
     expect(last_response.headers['X-Grape-After-Header']).to eq('1')
   end
-  
+
   it 'should returns all headers on error' do
     get '/error'
     expect(last_response.headers['X-Grape-Returns-Error']).to eq('1')