refactor: move json related header vars to internal (#1840)

* refactor: move json related header vars to internal

* refactor: use header.ContentType

rest/httpc/service_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/zeromicro/go-zero/rest/internal/header"
 )
 
 func TestNamedService_DoRequest(t *testing.T) {
@@ -43,7 +44,7 @@ func TestNamedService_DoRequestPost(t *testing.T) {
 	service := NewService("foo")
 	req, err := http.NewRequest(http.MethodPost, svr.URL, nil)
 	assert.Nil(t, err)
-	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set(header.ContentType, header.JsonContentType)
 	resp, err := service.DoRequest(req)
 	assert.Nil(t, err)
 	assert.Equal(t, http.StatusNotFound, resp.StatusCode)