Fix 500 error when no DisplayName in request body.

The original data of body from client to set acl like below:

<AccessControlPolicy xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <Owner>
    <ID>test:tester</ID>
    <DisplayName>test:tester</DisplayName>
  </Owner>
  <AccessControlList>
    ...
  </AccessControlList>
</AccessControlPolicy>

If the attribute "<DisplayName>" isn't exist, proxy will return
500 Internal Server Error.

Change-Id: Ia02af110109d5ecbcda05f707707b30e604af8f0
Closes-Bug: #1400367
Merge "Fix 500 error when no DisplayName in request body."