$_SERVER headers incorrectly read due to uppercase/lowercase mixup

When a Request object is created, the headers are set from $_SERVER.  All of the keys in $_SERVER are set to uppercase.  However, all other interactions with headers in Request assume, and force, lowercase.  This was preventing POST data from being read, since HttpRequest->getBodyData() is looking for the 'CONTENT_TYPE' header to determine how to decode data.  Simple one-line fix to force the $_SERVER keys to lowercase.