Hello,

I am working on this gem to provide backward-compatibility of the new `dig`
method in `Hash` and `Array`.

    https://rubygems.org/gems/ruby_dig

(See the accompanying blog post for more info.
http://development.invoca.com/ruby_dig-gem-adds-hash-dig-and-array-dig-from-ruby-2-3-to-earlier-versions/
)

But I can't tell from the documentation or tests in the original commit


https://github.com/ruby/ruby/commit/29862685c0acf3a40c6b1f9e8780cbbd86cba658

what is supposed to happen if Array#dig is passed a non-numeric index. For
example:

    ['zero', 'one', 'two'].dig(:first)

It seems like the C code is just calling the `[]` operator, in which case
it would raise this exception:

    TypeError: no implicit conversion of Symbol into Integer
But it definitely seems like the spirit of `dig` that `nil` should be
returned if the expected structure isn't there.  I believe the most typical
use case for `dig` is to dig through a JSON hash and either find the result
we expected, or get `nil`.  Wouldn't it defeat the purpose of `dig` if we
had to wrap it in a `rescue`?

Can we clarify the desired behavior, then update the documentation and
tests to reflect that?

-Colin