Restore visual diff to original behavior

This restores the following corner case:

When there is a difference, but it's only about
the case of the characters, don't want to show
the visual diff by default, but we offer the
feature to the user.

I also updated the tests the reflect the specs.
You are right.
2014.11.26. 10:30 ezt írta ("Aron Carroll" <notifications@github.com>):

> In h/static/scripts/directives/annotation.coffee:
>
> >          else
> >            t.hasDiff = false
> > -      @showDiff ?= @hasDiff or undefined
> > +      if @hasDiff
> > +        @showDiff ?= false
> > +      else
> > +        @showDiff ?= undefined
>
> I don't see the need for the else here, you're checking if something is
> defined, then if it's not you set it to undefined?
>
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> <https://github.com/hypothesis/h/pull/1717/files#r20923959>.
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