when {category|tag}_base include spaces their rewrite rules don't get generated correctly, resulting in 404's

[pbiron]

Steps to reproduce:

1. Go to Settings > Permalinks
2. Enter "this is a test" in the category_base field
3. Click "Save Changes"
4. Notice that category_base gets saved as "this%20is%20a%20test".
5. Notice that the rewrite rules such as the following get generated:

   'this%20is%20a%test/(.+?)?$' => 'index.php?%20was%$matches[1]&%20test%$matches[2]'
   

instead of (what I imagine the correct rule would be)

'this%20is%20a%test/(.+?)?$' => 'index.php?category_name=$matches[1]

I'm not sure whether the best thing to do is to disallow spaces in {category|tag}_base or to fixWP_Rewrite::generate_rewrite_rules() to generate correct rules if they do include spaces.