Use original `Array#sum` to speed up calculating

Use original `Array#sum` when calculating Numeric sum.
This commit is related #24804 issue.
Issue #24804 reports `Array#sum` becomes much slower when
ActiveSupport is included.

This commit tries to use original method as far as possible.

```shell

$ cat array_sum.rb
class Array
  alias core_sum sum
end

require 'benchmark/ips'
require 'active_support/core_ext/enumerable'

ary = [1.0] * 1_000_000

Benchmark.ips do |x|
  x.report("core sum") { ary.core_sum }
  x.report("AS's sum") { ary.sum }

   x.compare!
end

$ bundle exec ruby -v -I lib array_sum.rb
ruby 2.4.0dev (2016-05-01 master 54867) [x86_64-darwin14]
Calculating -------------------------------------
            core sum     4.000  i/100ms
            AS's sum     5.000  i/100ms
-------------------------------------------------
            core sum     50.492  (± 7.9%) i/s -    252.000
            AS's sum     50.116  (± 6.0%) i/s -    250.000

Comparison:
            core sum:       50.5 i/s
            AS's sum:       50.1 i/s - 1.01x slower
```

Signed-off-by: Jeremy Daer <jeremydaer@gmail.com>

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