substring, slice! should return nil without exception.

* array.c (rb_ary_insert): type fixed.

* string.c (rb_str_split_m): accept separator value nil as well.

* string.c (rb_str_become): was leaking memory.

* class.c (rb_include_module): should not alter other
  classes/modules by inclusion.  by this fix, local order may not
  be preserved for some cases.

* class.c (include_class_new): module may be T_ICLASS; retrieve
  original module information.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@2632 b2dd03c8-39d4-4d8f-98ff-823fe69b080e