Sets the subject of the mailing with the provided id.

Example

POST https://api.maileon.com/1.0/mailings/1/contents/subject
Content-type: application/vnd.maileon.api+xml
Accept: application/vnd.maileon.api+xml; charset=utf-8
Authorization: [XXX]

<?xml version="1.0" encoding="UTF-8"?>
<subject>New Brands Available!</subject>

Status: 204 No Content