fix PageList API arg passing to GeneratorList

PageList wasn't adding the 'g' prefix to API arguments it passes to GeneratorList, and GeneratorList doesn't do that itself. This fixes it. Note this affects the `site.categories` and `site.images` properties (as they use `PageList`).

To test, try this, before and after the change:

    #!/bin/python

    import mwclient
    site = mwclient.Site(('https', 'en.wikipedia.org'), '/w/')
    cats = site.categories
    for cat in cats:
        print(cat.name)

Note that `site.categories` is `listing.PageList(self, namespace=14)`. Without the fix, `site.categories` gives you all pages in Wikipedia (so ctrl-c it before you get banned...), with the fix, the `namespace` value is correctly passed through to `GeneratorList` as `gapnamespace=14` and you get the categories.