Fix space flag when Inf/NaN and width==3

	* sprintf.c (rb_str_format): while `"% 2f"` and `"% 4f"` result in
	  `" Inf"` and `" Inf"` respectively, `"% 3f"` results in
	  `"Inf"` (no space).
	Refactor "%f" % Inf/NaN

	* sprintf.c (rb_str_format): as for non-finite float, calculate
	  the exact needed size with the space flag.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@58625 b2dd03c8-39d4-4d8f-98ff-823fe69b080e