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

@@ -0,0 +1,47 @@
+require 'spec_helper'
+
+describe Grape::API do 
+ let(:error_header) do
+    Class.new(Grape::API) do
+      before do
+        header 'X-Grape-Before-Header', '1'
+      end
+      after do
+        header 'X-Grape-After-Header', '1'
+      end
+      get '/success' do
+        header 'X-Grape-Returns-Error', '1'
+      end
+      get '/error' do
+        header 'X-Grape-Returns-Error', '1'
+        error!({ success: false })
+      end
+    end
+  end
+
+  subject do
+    ErrorHeader = error_header unless defined?(ErrorHeader)
+    Class.new(Grape::API) do
+      format :json
+      mount ErrorHeader => '/'
+    end
+  end
+
+  def app
+    subject
+  end
+
+  it 'should returns all headers on success' do
+    get '/success'
+    expect(last_response.headers['X-Grape-Returns-Error']).to eq('1')
+    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')
+    expect(last_response.headers['X-Grape-Before-Header']).to eq('1')
+    expect(last_response.headers['X-Grape-After-Header']).to eq('1')
+  end
+end