Speed up Time.zone.now

@amatsuda, during his RailsConf talk this past year, presented a
benchmark that showed `Time.zone.now` (an Active Support joint)
performing 24.97x slower than Ruby's `Time.now`. Rails master appears to
be a _bit_ faster than that, currently clocking in at 18.25x slower than
`Time.now`. Here's the exact benchmark data for that:

```
Warming up --------------------------------------
            Time.now   127.923k i/100ms
       Time.zone.now    10.275k i/100ms
Calculating -------------------------------------
            Time.now      1.946M (± 5.9%) i/s -      9.722M in   5.010236s
       Time.zone.now    106.625k (± 4.3%) i/s -    534.300k in   5.020343s

Comparison:
            Time.now:  1946220.1 i/s
       Time.zone.now:   106625.5 i/s - 18.25x slower
```

What if I told you we could make `Time.zone.now` _even_ faster? Well,
that's exactly what this patch accomplishes. When creating `ActiveSupport::TimeWithZone`
objects, we try to convert the provided time to be in a UTC format. All
this patch does is, in the method where we convert a provided time to
UTC, check if the provided time is already UTC, and is a `Time` object
and then return early if that is the case, This sidesteps having to continue on,
and create a new `Time` object from scratch. Here's the exact benchmark
data for my patch:

```
Warming up --------------------------------------
            Time.now   124.136k i/100ms
       Time.zone.now    26.260k i/100ms
Calculating -------------------------------------
            Time.now      1.894M (± 6.4%) i/s -      9.434M in   5.000153s
       Time.zone.now    301.654k (± 4.3%) i/s -      1.523M in   5.058328s

Comparison:
            Time.now:  1893958.0 i/s
       Time.zone.now:   301653.7 i/s - 6.28x slower
```

With this patch, we go from `Time.zone.now` being 18.25x slower than
`Time.now` to only being 6.28x slower than `Time.now`. I'd obviously love some
verification on this patch, since these numbers sound pretty interesting... :)

This is the benchmark-ips report I have been using while working on this:

```ruby
require 'benchmark/ips'

Time.zone = 'Eastern Time (US & Canada)'

Benchmark.ips do |x|
  x.report('Time.now') {
    Time.now
  }

  x.report('Time.zone.now') {
    Time.zone.now
  }

  x.compare!
end
```

cc @amatsuda
cc performance folks @tenderlove and @schneems

![Pretty... pretty... pretty good.](https://media.giphy.com/media/bWeR8tA1QV4cM/giphy.gif)

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