Python 3: set webob.Response().body to a bytes value

In Python 3, the body attribute of a webob.Response instance has type 'bytes':

    >>> import webob
    >>> type(webob.Response().body)
    <class 'bytes'>

Change-Id: Ic60454dd1713ebd5ae84daaa113ccb0270ca37ed
Merge "Python 3: set webob.Response().body to a bytes value"
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