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
I think you mean: check what would happen with `\xa9`.
As a part of the selected data it shouldn't cause a problem because [\__str__() calls repr() on them](https://github.com/scrapy/scrapy/blob/36d99a0739059668b9cb3c84ad527d29a18687ca/scrapy/selector/unified.py#L150). 
But I will add another assertion for it too.