marshal.c: allow marshalling keyword_init struct

	struct.c: define rb_struct_s_keyword_init to shared with marshal.c

	internal.h: add the declaration to be used by marshal.c

	test/ruby/test_marshal.rb: add test for Bug#14314

	[Feature #14314] [ruby-core:84629]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62428 b2dd03c8-39d4-4d8f-98ff-823fe69b080e