Allow setting first-time-slug

In our application we allow users to set the slug.
This works fine for content that already exists (is persisted to DB), but when creating new content the slug generator will override the given slug resulting in duplicates.

lib/ecto_autoslug_field/slug_generator.ex

@@ -60,7 +60,7 @@ defmodule EctoAutoslugField.SlugGenerator do
     slug_builder = Keyword.get(opts, :slug_builder)
     always_change = Keyword.get(opts, :always_change, false)
 
-    slug_field = Map.get(changeset.data, slug_key)
+    slug_field = Ecto.Changeset.get_field(changeset, slug_key)
 
     if always_change == true or slug_field == nil do
       # We only generate slug on two occasions: