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.

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.

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