使用curl post数据CURLOPT_POSTFIELDS字符串和array的不同

curl CURLOPT_POSTFIELDS，可以填入字符串，数组，@文件地址
今天用到这个，发现一个问题，如果使用array, Content-Type会被赋值为multipart/form-data

CURLOPT_POSTFIELDS:
The full data to post in a HTTP “POST” operation. To post a file, prepend a filename with @ and use the full path. This can either be passed as a urlencoded string like ‘para1=val1&para2=val2&…’ or as an array with the field name as key and field data as value. If value is an array, the Content-Type header will be set to multipart/form-data.

正常情况应该是：

Content-Type: application/x-www-form-urlencoded