fix: Unexpected default rack.session

Some rack application fail when the `rack.session` already is
initialized, but with an unexpected value. For example, in an Rails
API-only application, unexpected behavior is triggered when a
`rack.session` is present.

This commit changes webmock, not to set a `rack.session` at all, similar
to how webserver do not set a session. The applications or a middleware
sets them up if needed.

Fixes bblimke#985

lib/webmock/rack_response.rb

@@ -52,22 +52,12 @@ def build_rack_env(request)
       end
       env['rack.url_scheme'] = uri.scheme
       env['rack.run_once']   = true
-      env['rack.session']    = session
-      env['rack.session.options'] = session_options
 
       headers.each do |k, v|
         env["HTTP_#{k.tr('-','_').upcase}"] = v
       end
 
       env
     end
-
-    def session
-      @session ||= {}
-    end
-
-    def session_options
-      @session_options ||= {}
-    end
   end
 end

spec/unit/rack_response_spec.rb

@@ -83,6 +83,17 @@
     end
   end
 
+  it "shouldn't set a rack.session" do
+    request = WebMock::RequestSignature.new(:get, 'www.example.com/env')
+    response = @rack_response.evaluate(request)
+
+    expect(response.status.first).to eq(200)
+    body = JSON.parse(response.body)
+
+    expect(body).not_to include("rack.session")
+    expect(body).not_to include("rack.session.options")
+  end
+
   describe 'rack error output' do
     before :each do
       @original_stderr = $stderr