Fix bug of Tempfile#size if nothing is written [Bug #13198]

	* lib/tempfile.rb (Tempfile#size): Fix its behavior when nothing
	  is written. Tempfile#size should return 0 in this case.
	  The patch is from nobu <nobu@ruby-lang.org>.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@58154 b2dd03c8-39d4-4d8f-98ff-823fe69b080e