Remove unused code and add specs for #open?

According to coveralls some parts of code is not longer used. I added
specs for untested code.

Gemfile.lock

@@ -26,7 +26,7 @@ GEM
       minitest (~> 5.1)
       tzinfo (~> 1.1)
     addressable (2.4.0)
-    ancestry (3.0.6)
+    ancestry (3.0.7)
       activerecord (>= 3.2.0)
     appraisal (2.2.0)
       bundler

lib/acts-as-messageable/model.rb

@@ -34,13 +34,8 @@ def acts_as_messageable(options = {})
         self.messages_class_name = options[:class_name].constantize
         messages_class_name.has_ancestry
 
-        if messages_class_name.respond_to?(:table_name=)
-          messages_class_name.table_name = options[:table_name]
-          messages_class_name.initialize_scopes
-        else
-          messages_class_name.set_table_name(options[:table_name])
-          ActiveSupport::Deprecation.warn("Calling set_table_name is deprecated. Please use `self.table_name = 'the_name'` instead.")
-        end
+        messages_class_name.table_name = options[:table_name]
+        messages_class_name.initialize_scopes
 
         messages_class_name.required = Array.wrap(options[:required])
         messages_class_name.validates_presence_of messages_class_name.required

lib/acts-as-messageable/rails3.rb

@@ -4,10 +4,6 @@ def initialize(subject)
       @subject = subject
     end
 
-    def attr_accessible(*args)
-      @subject.attr_accessible(*args)
-    end
-
     def default_scope(order_by)
       @subject.send(:default_scope, order(order_by))
     end

lib/acts-as-messageable/rails4.rb

@@ -4,10 +4,6 @@ def initialize(subject)
       @subject = subject
     end
 
-    def attr_accessible(*args)
-      @subject.attr_accessible(*args) if defined?(ProtectedAttributes)
-    end
-
     def default_scope(order_by)
       @subject.send(:default_scope) { order(order_by) }
     end

spec/acts-as-messageable_spec.rb

@@ -164,6 +164,7 @@
     it 'alice should have one unread message from bob' do
       expect(@alice.messages.are_from(@bob).unreaded.count).to eq(1)
       expect(@alice.messages.are_from(@bob).readed.count).to eq(0)
+      expect(@alice.messages.are_from(@bob).readed.all? { |message| message.open? }).to be_truthy
     end
 
     it 'alice should able to read message from bob' do