We need to set the content-type as well, or jenkins gets unhappy.

I figured this out by seeing what curl sends -- curl was working --
and comparing it to what nodejs was sending.

Auditors: ben

Test Plan:
Manually ran
    % telnet jenkins.khanacademy.org 80
    POST /buildByToken/buildWithParameters HTTP/1.1
    Host: jenkins.khanacademy.org
    Content-length: 129
    Content-Type: application/x-www-form-urlencoded
    Connection: keep-alive

    deploy-via-multijob&token=<token>&GIT_REVISION=bad&BUILD_USER_ID_FROM_SCRIPT=csilvers@khanacademy.org&cause=Sun+Wukong

and saw it respond 'Scheduled', rather than giving back a 500 error.