Add an alias of Kernel#yield_self named #then.

Author: dimameshcharakou

spec/tags/core/kernel/then_tags.txt

@@ -27,7 +27,6 @@ fails:Public methods on IO should not include encode_with
 fails:Public methods on IO should not include nonblock
 fails:Public methods on IO should not include nonblock=
 fails:Public methods on IO should not include nonblock?
-fails:Public methods on Kernel should include then
 fails:Public methods on Numeric should include clone
 fails:Public methods on Numeric should include dup
 fails:Public methods on Range should include %

spec/tags/truffle/methods_tags.txt

@@ -483,6 +483,8 @@ def yield_self
     end
   end
 
+  alias_method :then, :yield_self
+
   def test(cmd, file1, file2=nil)
     case cmd
     when ?d