Change Http::Cache::SPECIAL_KEYS from Array to Set

Slightly improves performance, for example, a simple benchmark:

```ruby
require 'benchmark/ips'
require 'set'

SPECIAL_KEYS = %w[extras no-cache max-age public must-revalidate]
SPECIAL_KEYS_SET = Set.new(SPECIAL_KEYS).freeze
directive = 'must-revalidate'

Benchmark.ips do |x|
  x.report('array') { SPECIAL_KEYS.include?(directive) }
  x.report('set') { SPECIAL_KEYS_SET.include?(directive) }
end
```

Output:

```
-------------------------------------
   array     67926 i/100ms
     set     74054 i/100ms
-------------------------------------
   array  2318423.4 (±2.8%) i/s -   11615346 in   5.014899s
     set  3387981.8 (±4.7%) i/s -   16958366 in   5.019355s
```
@rafaelfranca Makes it a little safer: if someone adds `<<` to `SPECIAL_KEYS` somewhere, it will be immediately noticed. `Set` wouldn't make sense in that unlikely scenario. At least that was my idea and interpretation for [other places](https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/http/headers.rb#L28) in Rails source where arrays/sets are frozen.