lastgenre: `force` config option doesn’t work
### Problem

I want to write the popular genres from Last.fm in the metatags of my music.

I have the following configuration:

```
lastgenre:
    whitelist: no
    count: 5
    fallback: nogenre
    min_weight: 20
    source: track
    force: yes
```

Now I run "beets lastgenre"

Other than what I had expected that beets would now write the 5 most popular genres into each file, it just outputs with (keep) and doesn't change anything to the genre metatag...

What could be going wrong here and how can I fix this?

Thanks a lot for any help!

diff --git a/beetsplug/lastgenre/__init__.py b/beetsplug/lastgenre/__init__.py
index 2f660206e..c4b973620 100644
--- a/beetsplug/lastgenre/__init__.py
+++ b/beetsplug/lastgenre/__init__.py
@@ -373,7 +373,7 @@ class LastGenrePlugin(plugins.BeetsPlugin):
         lastgenre_cmd = ui.Subcommand('lastgenre', help=u'fetch genres')
         lastgenre_cmd.parser.add_option(
             u'-f', u'--force', dest='force',
-            action='store_true', default=False,
+            action='store_true',
             help=u're-download genre when already present'
         )
         lastgenre_cmd.parser.add_option(
['beetsplug/lastgenre/__init__.py']