use defer file close in go client

Author: wing328

bin/configs/go-oneof-discriminator-lookup.yaml

@@ -1,6 +1,7 @@
 generatorName: go
 outputDir: samples/client/others/go/oneof-discriminator-lookup
 inputSpec: modules/openapi-generator/src/test/resources/3_0/go/spec-with-oneof-discriminator.yaml
+templateDir: modules/openapi-generator/src/main/resources/go
 additionalProperties:
   useOneOfDiscriminatorLookup: "true"
   hideGenerationTimestamp: "true"

modules/openapi-generator/src/main/resources/go/client.mustache

@@ -592,10 +592,7 @@ func addFile(w *multipart.Writer, fieldName, path string) error {
 	if err != nil {
 		return err
 	}
-	err = file.Close()
-	if err != nil {
-		return err
-	}
+	defer file.Close()
 
 	part, err := w.CreateFormFile(fieldName, filepath.Base(path))
 	if err != nil {