Raise if host field contains '/'

[wsanchez]

These two URLs render the same but are not the same:

URL(scheme=u'http', userinfo=u'', host=u'a', port=80, path=(u'c',), query=(), fragment=u'', rooted=True)
URL(scheme=u'http', userinfo=u'', host=u'a/c', port=80, path=(), query=(), fragment=u'', rooted=True)

Which is baffling until you look at all of the fields separately.