feign响应Image流对象

feign面向方法签名的http调用，越来越受欢迎，类型于rpc的thrift，只需要关注方法签名和返回值即可，当然feign响应流对象时，需要我们做一下特殊处理，否则会出现异常。也有一些文章通过重写HttpMessageConvert来实现，但我测试后发现还是失败的。
> Accept: image/*会返回406 Not Acceptable

### 解决方法
feign代码，注意返回值必须是`feign.Response`
```
  @PostMapping(value = "/wxa/getwxacodeunlimit")
  feign.Response getQrImagePath(@RequestParam("access_token") String accessToken,@RequestBody Map<String, Object> body);

  class WeixinClientFallback implements WeixinClient {
```
restcontroller代码,需要显示声明`produces`类型，这里因为是图片，所以声明为`MediaType.IMAGE_JPEG_VALUE`
```
 @GetMapping(value = "/v1/api/sales/qrcode", produces = MediaType.IMAGE_JPEG_VALUE)
  public void qrcode(HttpServletResponse response) {
    //获取token
    Map<String, Object> tokenMap = weixinClient.getToken("client_credential", wxAppId, wxAppSecret);
    if (tokenMap == null) {
      throw Exceptions.paramError("获取token失败");
    }
    if (tokenMap.containsKey("errcode")) {
      throw Exceptions.paramError(MapUtils.getString(tokenMap, "errmsg"));
    }
    String scene = String.format("salesPersonId=%s&agencyId=%s",
        TokenContext.getToken().getUserId(),
        TokenContext.getToken().getCompanyId());
    String token = MapUtils.getString(tokenMap, "access_token");
    Map<String, Object> requestParams = new HashMap<>();
    requestParams.put("scene", scene);
    requestParams.put("page", weixinPage);
    feign.Response imgReponse = weixinClient.getQrImagePath(token, requestParams);
    if (imgReponse == null) {
      throw Exceptions.paramError("获取二维码失败");
    }
    try (InputStream inputStream = imgReponse.body().asInputStream();
         ServletOutputStream outputStream = response.getOutputStream()) {
      response.setContentType("image/png");
      outputStream.write(IOUtils.toByteArray(inputStream));
    } catch (RuntimeException e) {
      throw Exceptions.paramError("获取二维码失败");
    } catch (Exception ex) {
      throw Exceptions.paramError("获取二维码失败");
    }
  }
```