code: protected void flatCopyTo(String prefix, Map<String, Object> sourceMap, HttpHeaders headers) {
          for (Map.Entry<String, Object> entry : sourceMap.entrySet()) {
              String key = prefix + entry.getKey();
              Object value = entry.getValue();
              if (value instanceof String) {
                  addToHeader(headers, key, (CharSequence) value);
              } else if (value instanceof Number) {
                  addToHeader(headers, key, value.toString());
              } else if (value instanceof Map) {
                  flatCopyTo(key + ".", (Map<String, Object>) value, headers);
              }
          }
      }
docs: 扁平化复制
  
  @param prefix    前缀
  @param sourceMap 原始map
  @param headers   目标map