Add missing Content-MD5 header to the POST Object Restore request

I tried to write a unit test for this change but unfortunately AWS takes an indefinite amount of time to transition objects to glacier even when you set 0 days which makes automating a restore test case difficult.

I tested this manually though and sniffed the request/response to show its setting the header now and that AWS accepted the request. Note: I modified the Authorization header when I pasted it below.

    POST /meta?restore HTTP/1.1.
    Host: ozaintbucket.s3.amazonaws.com.
    Accept-Encoding: identity.
    Date: Mon, 03 Feb 2014 02:29:34 GMT.
    Content-Length: 155.
    User-Agent: Boto/2.23.0 Python/2.7.3 Linux/3.9.10-100.fc17.x86_64.
    Authorization: AWS BLAH:BLAH.
    Content-MD5: 52ovPBx7I1yaJo86KXcVdQ==.
    .
    <?xml version="1.0" encoding="UTF-8"?>
          <RestoreRequest xmlns="http://s3.amazonaws.com/doc/2006-03-01">
            <Days>1</Days>
          </RestoreRequest>
    #
    T 205.251.242.184:80 -> 10.181.164.198:49398 [A]
    ......
    #
    T 205.251.242.184:80 -> 10.181.164.198:49398 [AP]
    HTTP/1.1 202 Accepted.
    x-amz-id-2: Tr0my0QQoMKf3Ru7pym0b+8mtHQIBR5ZqOuiugYh+5dEl4SzQriEIlRKk1N+JGEc.
    x-amz-request-id: 9460B322F2BFCF8C.
    Date: Mon, 03 Feb 2014 02:29:35 GMT.
    Content-Length: 0.
    Server: AmazonS3.
    .