/**
  * 设置字段请求头信息
  *
  * @param field 字段请求头信息{@link com.lidroid.xutils.http.client.multipart.MinimalField}
  */
 public void setField(final MinimalField field) {
   if (field == null) {
     return;
   }
   String key = field.getName().toLowerCase(Locale.US);
   List<MinimalField> list = fieldMap.get(key);
   if (list == null || list.isEmpty()) {
     addField(field);
     return;
   }
   list.clear();
   list.add(field);
   int firstOccurrence = -1;
   int index = 0;
   for (Iterator<MinimalField> it = this.fields.iterator(); it.hasNext(); index++) {
     MinimalField f = it.next();
     if (f.getName().equalsIgnoreCase(field.getName())) {
       it.remove();
       if (firstOccurrence == -1) {
         firstOccurrence = index;
       }
     }
   }
   this.fields.add(firstOccurrence, field);
 }
 /**
  * 添加字段请求头信息
  *
  * @param field 字段请求头信息{@link com.lidroid.xutils.http.client.multipart.MinimalField}
  */
 public void addField(final MinimalField field) {
   if (field == null) {
     return;
   }
   String key = field.getName().toLowerCase(Locale.US);
   List<MinimalField> values = this.fieldMap.get(key);
   if (values == null) {
     values = new LinkedList<MinimalField>();
     this.fieldMap.put(key, values);
   }
   values.add(field);
   this.fields.add(field);
 }