abp vnext请求头增加，以及请求头增加公共头部回调

context.Services.AddHttpContextAccessor();

要访问请求头的话，要加上这个语句；

• 追加请求头的方法：
  1、服务层服务的HttpApi.Client项目Module类的ConfigureServices方法开头位置添加如下代码：
  context.Services.AddTransient<AddHeaderHandler>();
  context.Services.AddHttpClient(ProductManagerRemoteServiceConsts.RemoteServiceName)
  　　.AddHttpMessageHandler<AddHeaderHandler>();
  
  在一个JGSoftAbpClientModule添加AddHeaderHandler类如下：
  public class AddHeaderHandler : DelegatingHandler, ITransientDependency
  {
  private ICurrentUserExtension_currentUserExtension;
  public AddHeaderHandler(ICurrentUserExtension currentUserExtension)
  {
  _currentUserExtension = currentUser;
  }
  protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
  CancellationToken cancellationToken)
  {
  var headers =request.Headers;
  if (!headers.Contains("Systemno"))
  {
  headers.Add("Systemno",_currentUserExtension.Systemno);
  }
  return await base.SendAsync(request, cancellationToken);
  }
  }