Fix 'rack.errors' not being set for Rack apps

lib/webmock/rack_response.rb

@@ -44,6 +44,7 @@ def build_rack_env(request)
 
       # Rack-specific variables
       env['rack.input']      = StringIO.new(body)
+      env['rack.errors']     = $stderr
       env['rack.version']    = Rack::VERSION
       env['rack.url_scheme'] = uri.scheme
       env['rack.run_once']   = true

spec/support/my_rack_app.rb

@@ -2,7 +2,7 @@
 
 class MyRackApp
   class NonArrayResponse
-    # The rack response body need not implement #join, 
+    # The rack response body need not implement #join,
     # but it must implement #each.  It need not be an Array.
     # ActionDispatch::Response, for example, exercises that fact.
     # See: http://rack.rubyforge.org/doc/SPEC.html
@@ -32,6 +32,9 @@ def self.call(env)
         else
           [401, {}, [""]]
         end
+      when ['GET', '/error']
+        env['rack.errors'].puts('Error!')
+        [500, {}, ['']]
       else
         [404, {}, ['']]
     end

spec/unit/rack_response_spec.rb

@@ -49,6 +49,24 @@
     response.body.should include('Good to meet you, Jimmy!')
   end
 
+  describe 'rack error output' do
+    before :each do
+      @original_stderr = $stderr
+      $stderr = StringIO.new
+    end
+
+    after :each do
+      $stderr = @original_stderr
+    end
+
+    it 'should behave correctly when an app uses rack.errors' do
+      request = WebMock::RequestSignature.new(:get, 'www.example.com/error')
+
+      expect { @rack_response.evaluate(request) }.to_not raise_error
+      expect($stderr.length).to_not eq 0
+    end
+  end
+
   describe 'basic auth request' do
     before :each do
       @rack_response_with_basic_auth = WebMock::RackResponse.new(