Ensure to_param always returns a string

URL helpers assume to_param returns a string and raise a type error if
this is not true.

Signed-off-by: Jose Fernandez <jose@umn.edu>

lib/salty_slugs.rb

@@ -63,7 +63,7 @@ def sluggify(text)
 
       module InstanceMethods
         def to_param
-          return self.id if slug_prepend_id && self[slug_column].blank?
+          return self.id.to_s if slug_prepend_id && self[slug_column].blank?
           slug_prepend_id ? "#{self.id}-#{self[slug_column]}" : self[slug_column]
         end
       end

test/slug_test.rb

@@ -74,6 +74,8 @@ def test_sync_slug
   def test_to_param
     assert_equal @post.to_param, "#{@post.id}-#{@post[Post.slug_column]}"
     assert_equal @product.to_param, "#{@product[Product.slug_column]}"
+    post_with_blank_slug = Post.create
+    assert_equal post_with_blank_slug.to_param, post_with_blank_slug.id.to_s
   end
 
   def test_sluggify