slug should replace underscores

`String#parameterize` already hyphenates, but it doesn't convert underscores since they're url-valid.

Added a spec for this regression.

~~~ ruby
    # Before
    Slug.for("o-o_o-o") #=> "o-o_o-o"

    # After
    Slug.for("o-o_o-o") #=> "o-o-o-o"
~~~