This makes `datetime.serialize` about 10% faster.

```ruby
type = ActiveRecord::Type.lookup(:datetime)
time = Time.now.utc

Benchmark.ips do |x|
  x.report("type.serialize(time)") do
    type.serialize(time)
    type.serialize(time)
    type.serialize(time)
    type.serialize(time)
  end
end
```

Before:

```
Warming up --------------------------------------
type.serialize(time)    12.899k i/100ms
Calculating -------------------------------------
type.serialize(time)    131.293k (± 1.6%) i/s -    657.849k in   5.011870s
```

After:

```
Warming up --------------------------------------
type.serialize(time)    14.603k i/100ms
Calculating -------------------------------------
type.serialize(time)    145.941k (± 1.1%) i/s -    730.150k in   5.003639s
```