Fixed formatted substring expansion [Bug #16767]

	---
	 strftime.c             | 2 ++
	 test/ruby/test_time.rb | 7 +++++++
	 2 files changed, 9 insertions(+)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67916 b2dd03c8-39d4-4d8f-98ff-823fe69b080e