Minor DRY refactor.

Author: jdantonio

lib/concurrent/future.rb

@@ -88,8 +88,6 @@ def set(value = IVar::NO_VALUE, &block)
       execute
     end
 
-    protected :complete
-
     private
 
     # @!visibility private

lib/concurrent/ivar.rb

@@ -103,11 +103,8 @@ def add_observer(observer = nil, func = :update, &block)
     #   been set or otherwise completed
     # @return [IVar] self
     def set(value = NO_VALUE)
-      if (block_given? && value != NO_VALUE) || (!block_given? && value == NO_VALUE)
-        raise ArgumentError.new('must set with either a value or a block')
-      elsif ! compare_and_set_state(:processing, :pending)
-        raise MultipleAssignmentError
-      end
+      check_for_block_or_value!(block_given?, value)
+      raise MultipleAssignmentError unless compare_and_set_state(:processing, :pending)
 
       begin
         value = yield if block_given?