A bug in ImageFileCollection.filter

[RuiningZHAO]

There is a bug in ImageFileCollection.filter.

class ImageFileCollection:
    ...
    def filter(self, **kwd):
        ...
        files = self.files_filtered(include_path=True, **kwd)
        return ImageFileCollection(filenames=files,
                                   keywords=self.keywords)

The call of ImageFileCollection here should be with arg ext, otherwise the keywords not in the first extension will become -- in the returned collection.

[mwcraig]

Thanks for reporting this, @RuiningZHAO -- I'll try to get to this in the next couple of weeks, but I'd be happy to review a PR to fix it sooner than that :)