Fix bug in sequencing slugs starting with numbers

Fixes a bug where a slug conflicting with a slug that starts with a number
would get incorrectly sequenced based on the number at the start of the slug,
rather than as a fresh sequence.

lib/friendly_id/sequentially_slugged.rb

@@ -32,15 +32,13 @@ def next_slug
     private
 
       def next_sequence_number
-        if last_sequence_number == 0
-          2
-        else
-          last_sequence_number + 1
-        end
+        last_sequence_number ? last_sequence_number + 1 : 2
       end
 
       def last_sequence_number
-        slug_conflicts.last.split("#{slug}#{sequence_separator}").last.to_i
+        if match = /#{slug}#{sequence_separator}(\d+)/.match(slug_conflicts.last)
+          match[1].to_i
+        end
       end
 
       def slug_conflicts

test/sequentially_slugged_test.rb

@@ -64,6 +64,15 @@ def model_class
     end
   end
 
+  test "should correctly sequence slugs that begin with a number" do
+    transaction do
+      record1 = model_class.create! :name => "2010 to 2015 Records"
+      assert_equal "2010-to-2015-records", record1.slug
+      record2 = model_class.create! :name => "2010 to 2015 Records"
+      assert_equal "2010-to-2015-records-2", record2.slug
+    end
+  end
+
   test "should sequence with a custom sequence separator" do
     model_class = Class.new(ActiveRecord::Base) do
       self.table_name = "novelists"