Change `timestamp_attributes_for_{create,update}` from symbol to string

`timestamp_attributes_for_{create,update}` is defined as symbol but
always used as string with `to_s`. This allocates extra strings. To
avoid extra allocation, change the definitions from symbol to string.

```ruby
pp ObjectSpace::AllocationTracer.trace {
  1_000.times { |i|
    Post.create!
  }
}
```

Before:

```
["~/rails/activerecord/lib/active_record/timestamp.rb", 121]=>[1002, 0, 750, 0, 1, 18528],
["~/rails/activerecord/lib/active_record/timestamp.rb", 105]=>[1002, 0, 750, 0, 1, 7720],
["~/rails/activerecord/lib/active_record/timestamp.rb", 101]=>[1002, 0, 750, 0, 1, 7720],
["~/rails/activerecord/lib/active_record/timestamp.rb", 109]=>[1002, 0, 750, 0, 1, 13896],
["~/rails/activerecord/lib/active_record/timestamp.rb", 61]=>[4008, 0, 3000, 0, 1, 30880],
```

After:

```
["~/rails/activerecord/lib/active_record/timestamp.rb", 120]=>[1000, 0, 756, 0, 1, 17184],
["~/rails/activerecord/lib/active_record/timestamp.rb", 104]=>[1000, 0, 756, 0, 1, 7160],
["~/rails/activerecord/lib/active_record/timestamp.rb", 100]=>[1000, 0, 756, 0, 1, 7160],
["~/rails/activerecord/lib/active_record/timestamp.rb", 108]=>[1000, 0, 756, 0, 1, 12888],
```

0 files changed, 0 insertions, 0 deletions