Allow per-field overriding of default_operator

[cript0nauta]

Hi! I'm thinking in using this library to refactor an existing RESTful API. I need to be able to override the default_operator attribute of some fields but not of all of them, but I noted that is not possible yet (the column_overrides doesn't use this). I wrote a simple fix for this issue.

With this change I could do something like this:

class AlbumFilterSet(FilterSet):
    class Meta:
        model = Album
        query = session.query(Album)
        operators = (Equal, Greater, Less)
        column_overrides = {
            'name': {'default_operator': Like}
        }

[llazzaro]

@jmcarp Any news on this fix? I also hit the same bug.