Tweak PR a bit

By using `query.Values` we are using the URL tags and are omitting any
empty values. This seems to be inline with what the API expects. Could
you both this this version @dheitman42 and @zbindenren?

project_import_export.go

@@ -18,14 +18,14 @@ package gitlab
 
 import (
 	"bytes"
-	"encoding/json"
 	"fmt"
 	"io"
 	"mime/multipart"
 	"net/http"
 	"os"
-	"strings"
 	"time"
+
+	"github.com/google/go-querystring/query"
 )
 
 // ProjectImportExportService handles communication with the project
@@ -184,32 +184,37 @@ func (s *ProjectImportExportService) ImportFile(opt *ImportFileOptions, options
 		return nil, nil, err
 	}
 	defer file.Close()
-	var b bytes.Buffer
-	writer := multipart.NewWriter(&b)
-	var optMap map[string]interface{}
-	optMarshalled, err := json.Marshal(opt)
+
+	body := new(bytes.Buffer)
+	writer := multipart.NewWriter(body)
+
+	fields, err := query.Values(opt)
 	if err != nil {
 		return nil, nil, err
 	}
-	json.Unmarshal(optMarshalled, &optMap)
 
-	for key, value := range optMap {
-		if strings.ToLower(key) == "file" {
+	for name := range fields {
+		if name == "file" {
 			part, err := writer.CreateFormFile("file", file.Name())
 			if err != nil {
 				return nil, nil, err
 			}
-			_, err = io.Copy(part, file)
+			if _, err = io.Copy(part, file); err != nil {
+				return nil, nil, err
+			}
 		} else {
-			_ = writer.WriteField(strings.ToLower(key), fmt.Sprintf("%v", value))
+			if err = writer.WriteField(name, fmt.Sprintf("%v", fields.Get(name))); err != nil {
+				return nil, nil, err
+			}
 		}
 	}
 
 	err = writer.Close()
 	if err != nil {
 		return nil, nil, err
 	}
-	req, err := s.client.NewRequest(http.MethodPost, "projects/import", &b, options)
+
+	req, err := s.client.NewRequest(http.MethodPost, "projects/import", body, options)
 	if err != nil {
 		return nil, nil, err
 	}