POST https://www.googleapis.com/batch
Content-Type: multipart/mixed; boundary="foo_bar"
Authorization: Bearer {ACCESS_TOKEN}

--foo_bar
Content-Type: application/http

GET /gmail/v1/users/me/messages/152d10540c21bd07

--foo_bar
Content-Type: application/http

GET /gmail/v1/users/me/messages/152d1050d666d7ad

--foo_bar--

--batch_7Xp52oGIwpA_AAEAc7ERnGU
Content-Type: application/http

HTTP/1.1 200 OK
ETag: "A-DdBGA6g-wV4rIZCu5Hcm3JQpY/w2hzEg9kqXFH7AEJ-oSc-y10HNQ"
Content-Type: application/json; charset=UTF-8
Date: Thu, 11 Feb 2016 16:02:06 GMT
Expires: Thu, 11 Feb 2016 16:02:06 GMT
Cache-Control: private, max-age=0
Content-Length: 2809

{
 "id": "152d10540c21bd07",
 "threadId": "152d1050d666d7ad",
 "labelIds": [
  "SENT",
  "INBOX",
  "IMPORTANT"
 ],
 "snippet": "Likewise buddy.", ...
}

--batch_7Xp52oGIwpA_AAEAc7ERnGU
Content-Type: application/http

HTTP/1.1 200 OK
ETag: "A-DdBGA6g-wV4rIZCu5Hcm3JQpY/7v2nqQFBDmEHVvEQoboiwSidilE"
Content-Type: application/json; charset=UTF-8
Date: Thu, 11 Feb 2016 16:02:06 GMT
Expires: Thu, 11 Feb 2016 16:02:06 GMT
Cache-Control: private, max-age=0
Content-Length: 1752

{
 "id": "152d1050d666d7ad",
 "threadId": "152d1050d666d7ad",
 "labelIds": [
  "SENT",
  "INBOX",
  "IMPORTANT"
 ],
 "snippet": "Nice to meet you.", ...
}

--batch_7Xp52oGIwpA_AAEAc7ERnGU--