Fix Enumerable#sum redefined warning

If we require 'active_support/core_ext/enumerable' on Ruby 2.4,
we'll see following warning because `Enumerable#sum` and `Array#sum`
are added in Ruby 2.4.

```
rails/rails/activesupport/lib/active_support/core_ext/enumerable.rb:20: warning: method redefined; discarding old sum
```

The minimal way to fix the warning is `alias sum sum`.

```
$ ruby -v
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]

$ ruby -w -e "def a; end; def a; end"
-e:1: warning: method redefined; discarding old a
-e:1: warning: previous definition of a was here

$ ruby -w -e "def a; end; alias a a; def a; end"
```

But this behavior is not intended. (@amatsuda was told by @ko1)
So we should use `alias` as a meaningful way.

Ruby 2.4's `sum`  (`orig_sum`) assumes an `identity` is `0` when we omit `identity`
so we can delegate to `orig_sum` with explicit `identity` only.
In a strict sense, we can detect `identity` by check instance's class
but we don't care at this time about that because calling `Enumerable#sum` is rare.
In many cases, we will call `Array#sum`.

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