Fix controller attribute access.

This fixes the access to the controller attribute, making it clear that
the setter should be a `private` internal API. This was reflected
previously in the YARD doc, but that did not match the code.

Author: cupakromer

lib/rspec/rails/view_rendering.rb

@@ -8,7 +8,11 @@ module ViewRendering
 
       # @!attribute [r]
       # Returns the controller object instance under test.
-      attr_accessor :controller
+      attr_reader :controller
+
+      # @private
+      attr_writer :controller
+      private :controller=
 
       # DSL methods
       module ClassMethods