Use actual instance directly

Author: cosmo0920

spec/lib/fluent/plugin/out_aws-elasticsearch-service_spec.rb

@@ -16,19 +16,19 @@
     end
 
     it "`endpoint` is array." do
-      instance.instance_variable_get(:@endpoint).map do |ep|
+      instance.endpoint.map do |ep|
         expect(ep[:url]).to eq "xxxxxxxxxxxxxxxxxxxx"
       end
     end
 
     it "should get region" do
-      instance.instance_variable_get(:@endpoint).map do |ep|
+      instance.endpoint.map do |ep|
         expect(ep[:region]).to eq "us-east-1"
       end
     end
 
     it "should get default values" do
-      instance.instance_variable_get(:@endpoint).map do |ep|
+      instance.endpoint.map do |ep|
         expect(ep[:access_key_id]).to eq ""
         expect(ep[:secret_access_key]).to eq ""
         expect(ep[:assume_role_arn]).to eq nil