[FIX] v2.ODataModel: etag update fails

- as header fields are case-insensitive the etag update fails due to 
incorrect access to the response headers

Change-Id: I2611f703f945f35005aeb46cdb110ff90447541a
CR-Id: 012006153200015478942014
Merge "[FIX] v2.ODataModel: etag update fails" into rel-1.26