merge revision(s) r45534: [Backport #9709]

	* string.c (str_buf_cat): should round up the capacity by 4KiB,
	  but not number of rooms.  [ruby-core:61886] [Bug #9709]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46187 b2dd03c8-39d4-4d8f-98ff-823fe69b080e