Make repo creation for API similar to UI (#6142)

routers/api/v1/repo/repo.go

 
 // CreateUserRepo create a repository for a user
 func CreateUserRepo(ctx *context.APIContext, owner *models.User, opt api.CreateRepoOption) {
+	if opt.AutoInit && opt.Readme == "" {
+		opt.Readme = "Default"
+	}
 	repo, err := models.CreateRepository(ctx.User, owner, models.CreateRepoOptions{
 		Name:        opt.Name,
 		Description: opt.Description,