update_object_meta replaces previously added headers

[tomplus]

Hello.

As I can see in the example in the documentation: https://www.alibabacloud.com/help/en/object-storage-service/latest/manage-object-metadata-1

after run this example

bucket.update_object_meta(key, {'x-oss-meta-author': 'O. Henry'})
bucket.update_object_meta(key, {'x-oss-meta-price': '100 dollar'})

I expect to have two headers x-oss-meta-author, x-oss-meta-price but only last one exists.

It looks like it's caused by this directive:

aliyun-oss-python-sdk/oss2/api.py

Line 1112 in fadfabb

headers[OSS_METADATA_DIRECTIVE] = 'REPLACE'

introduced by 7769f1e

Can this directive be removed or exposed as an optional argument? I'm ready to create a PR if you wish.

Thanks.