Do not mark attribute as changed if value is the same

[shioyama]

(Note: I'm starting to post bug fixes and important changes as pull requests to
make them more visible.)

Currently, for a model post with dirty tracking enabled, this is what happens:

post = Post.new
post.title        #=> nil
post.changed?     #=> false
post.changed      #=> []
post.changes      #=> {}

So far so good, but if we set the value to nil:

post.title = nil
post.changed?     #=> true

This is a bug, which this PR fixes. After the change:

post.title = nil
post.changed?   #=> false
post.title = "foo"
post.changed?   #=> true

Note: the change applies to both ActiveModel and Sequel modules.

[shioyama]

While working on this fix I noticed that there is a tricky relationship between dirty tracking and fallbacks. If I have a model Article with fallbacks enabled:

class Article
  include Mobility
  translates :title, dirty: true, fallbacks: { en: 'ja' }
end

and I set the value in Japanese, the value in English will fall through to this value:

article = Article.new
article.title
#=> nil
Mobility.with_locale(:ja) { article.title = "タイトル" }
article.title
#=> "タイトル"

In the dirty module, when we check the current value of the attribute in the current locale, we need to disable fallbacks, otherwise we will compare with the fallback value and mark a change when we set to the same blank value:

article.title = nil
article.changed
#=> ["title_ja", "title_en"]

We don't want this - i.e., while I may want to fall through to a different locale, I don't want to compare with the value in that locale when marking attributes in the current locale as having changed or not.

If instead we disable fallbacks (read(locale, options.merge(fallbacks: false))), then we get what (I think) you'd expect:

article.title = nil
article.changed
#=> ["title_ja"]

Only the value in Japanese has changed (from nil to タイトル).

However, when we do update the value, do we mark the change as a change from nil or as a change from the fallback value?

i.e.

article.title = "title"
article.changes
#=> { "title_ja" => [nil, "タイトル"], "title_en" => ["タイトル", "title"] }

Or should it be marked as a change from nil to the new value?

I have gone with the former (change from fallback value), becuase: a) to make it do otherwise would require hacking hte internals of Rails/Sequel, which I don't want to do, b) I think intuitively, this sort of makes sense.