selector.__repr__ test

Some may consider it redundant, I won't argue.
But discussions and pull requests on selector's methods come and go these days (referring to extract_one, join etc) so I thought some extra coverage may prove useful later.
Which means I have a patch in mind and I need some test coverage before working on it and opening a PR
`__repr__` must return a bytestring in Python 2.x, not unicode, and for maximum compatibility it should be ascii-only. 

So 'u' should be removed, and I think it is better to check what happened with `'\xa9'`.