Fix self value in a block is changed with return value for Fixnum, nil, instance variable.

Fix #1504

test/t/proc.rb

@@ -60,13 +60,34 @@ assert('Proc#return_does_not_break_self') do
     attr_accessor :block
     def initialize
     end
-    def register_block
+    def return_array
       @block = Proc.new { self }
       return []
     end
+    def return_instance_variable
+      @block = Proc.new { self }
+      return @block
+    end
+    def return_const_fixnum
+      @block = Proc.new { self }
+      return 123
+    end
+    def return_nil
+      @block = Proc.new { self }
+      return nil
+    end
   end
 
   c = TestClass.new
-  assert_equal [], c.register_block
+  assert_equal [], c.return_array
+  assert_equal c, c.block.call
+
+  c.return_instance_variable
+  assert_equal c, c.block.call
+
+  assert_equal 123, c.return_const_fixnum
+  assert_equal c, c.block.call
+
+  assert_equal nil, c.return_nil
   assert_equal c, c.block.call
 end