Case-insensitivity of keys doesn't extend to set operations

Would be nice to have this work correctly:
```
>>> d = multidict.CIMultiDict()
>>> d["CONTENT-LENGTH"] = "4"
>>> "Content-Length" in d.keys()
True
>>> {"Content-Length"} & d.keys()
set()  # Should get a result here
>>> {"CONTENT-LENGTH"} & d.keys()
{'CONTENT-LENGTH'}
```

I tried to do this in aiohttp to check if any of a set of headers is present.