Google Cloud Storage JSON API: batch delete returns Unknown Error (500 Internal Server Error)

I'm trying to delete several object in batch using the documentation: Sending Batch Requests. Here is my request (<my_api_key> is the valid API key and other methods like list contents of a bucket work good, <my_bucket> is the placeholder for the exact bucket name):

POST /batch?key=<my_api_key>
host: www.googleapis.com
content-type:multipart/mixed; boundary="===============7330845974216740156=="

--===============7330845974216740156==
Content-Type: application/http 
Content-Transfer-Encoding:binary

DELETE /storage/v1/b/<my_bucket>/o/James-Hetfield-happy.jpg
--===============7330845974216740156==

Here is the response:

access-control-allow-credentials:true
access-control-allow-origin:chrome-extension://fdmmgilgnpjigdojojpjoooidkmcomcm
access-control-expose-headers:Cache-Control,Content-Encoding,Content-Length,Content-Type,Date,Expires,Pragma,Server,Vary
alternate-protocol:443:quic,p=0.02
cache-control:no-cache, no-store, max-age=0, must-revalidate
content-encoding:gzip
content-length:33
content-type:text/html; charset=UTF-8
date:Tue, 10 Feb 2015 16:55:14 GMT
expires:Fri, 01 Jan 1990 00:00:00 GMT
pragma:no-cache
server:GSE
status:500 Internal Server Error
vary:Origin
vary:X-Origin
version:HTTP/1.1
x-content-type-options:nosniff
x-frame-options:SAMEORIGIN
x-xss-protection:1; mode=block

Unknown Error

When I send the following body:

--===============7330845974216740156==
DELETE /storage/v1/b/<my_bucket>/o/James-Hetfield-happy.jpg
--===============7330845974216740156==

I got 200 OK response with the body --batch_rTJhZwR1jHM_AAh2WtGp7ik-- but the file still exists.

Please advise proper format for sending batch delete requests using Google Cloud Storage JSON API.