* Make it correctly handle lambdas
* Make it iterate over the block if block is given

The original implementation was flawed, based on lazy_set_method
instead of lazy_add_method.

Note that there is no implicit map when passing a block, the return
value of the block passed to with_index is ignored, just as it
is for Enumerator#with_index. Also like Enumerator#with_index,
when called with a block, the return value is an enumerator without
the index.

Fixes [Bug #16414]