Python 3: enable tests/unit/middleware/test_request_id.py

In Python 3, webob will return bytes, so we need to fix generate_request_id()
so that it also returns bytes. The behaviour is unchanged in Python 2.

Change-Id: I389cf20e4c999c9bb576a42388497852f8ba27ec
Merge "Python 3: enable tests/unit/middleware/test_request_id.py"