Blockless Struct#select should return an enumerator

Discovered this gap trying to get 'test_enumerator#test_size_for_enum_created_from_struct` passing for #980

For 1.9+ blockless `Struct#select` returns an enumerator.  Note that the latest RubySpect contains a [test covering this](https://github.com/rubyspec/rubyspec/blob/93d0ec63e14f6e88214c6c9b0f6cbf52e6d6379e/core/struct/select_spec.rb#L29).