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

iainbeeston/rails,bradleypriest/rails,kmayer/rails,yahonda/rails,rails/rails,prathamesh-sonpatki/rails,arunagw/rails,illacceptanything/illacceptanything,gcourtemanche/rails,voray/rails,flanger001/rails,kmcphillips/rails,tjschuck/rails,illacceptanything/illacceptanything,prathamesh-sonpatki/rails,yalab/rails,felipecvo/rails,prathamesh-sonpatki/rails,mathieujobin/reduced-rails-for-travis,elfassy/rails,kmayer/rails,travisofthenorth/rails,samphilipd/rails,vipulnsward/rails,MichaelSp/rails,matrinox/rails,gavingmiller/rails,jeremy/rails,yawboakye/rails,88rabbit/newRails,vipulnsward/rails,fabianoleittes/rails,assain/rails,Stellenticket/rails,amoody2108/TechForJustice,bradleypriest/rails,tjschuck/rails,BlakeWilliams/rails,EmmaB/rails-1,ttanimichi/rails,lucasmazza/rails,pschambacher/rails,arjes/rails,mohitnatoo/rails,yhirano55/rails,esparta/rails,bolek/rails,marklocklear/rails,amoody2108/TechForJustice,yhirano55/rails,yalab/rails,ledestin/rails,hanystudy/rails,kaspth/rails,yasslab/railsguides.jp,coreyward/rails,joonyou/rails,iainbeeston/rails,kirs/rails-1,kachick/rails,esparta/rails,MSP-Greg/rails,notapatch/rails,sergey-alekseev/rails,yasslab/railsguides.jp,riseshia/railsguides.kr,kenta-s/rails,riseshia/railsguides.kr,utilum/rails,yawboakye/rails,mtsmfm/railstest,repinel/rails,rbhitchcock/rails,yahonda/rails,kddeisz/rails,repinel/rails,betesh/rails,yasslab/railsguides.jp,illacceptanything/illacceptanything,kmcphillips/rails,ngpestelos/rails,marklocklear/rails,voray/rails,shioyama/rails,yasslab/railsguides.jp,gcourtemanche/rails,coreyward/rails,utilum/rails,bogdanvlviv/rails,schuetzm/rails,MichaelSp/rails,bogdanvlviv/rails,coreyward/rails,matrinox/rails,Erol/rails,arunagw/rails,vipulnsward/rails,deraru/rails,illacceptanything/illacceptanything,mechanicles/rails,aditya-kapoor/rails,aditya-kapoor/rails,prathamesh-sonpatki/rails,gauravtiwari/rails,notapatch/rails,koic/rails,lucasmazza/rails,pschambacher/rails,felipecvo/rails,ledestin/rails,travisofthenorth/rails,amoody2108/TechForJustice,yawboakye/rails,fabianoleittes/rails,elfassy/rails,yhirano55/rails,kddeisz/rails,mtsmfm/railstest,xlymian/rails,illacceptanything/illacceptanything,starknx/rails,gavingmiller/rails,arunagw/rails,kaspth/rails,assain/rails,fabianoleittes/rails,pvalena/rails,palkan/rails,brchristian/rails,hanystudy/rails,illacceptanything/illacceptanything,shioyama/rails,gavingmiller/rails,stefanmb/rails,koic/rails,repinel/rails,Erol/rails,rafaelfranca/omg-rails,gfvcastro/rails,rails/rails,EmmaB/rails-1,mohitnatoo/rails,ledestin/rails,flanger001/rails,voray/rails,flanger001/rails,yalab/rails,deraru/rails,tjschuck/rails,brchristian/rails,illacceptanything/illacceptanything,richseviora/rails,Erol/rails,yahonda/rails,tgxworld/rails,illacceptanything/illacceptanything,betesh/rails,gauravtiwari/rails,mijoharas/rails,jeremy/rails,ngpestelos/rails,MSP-Greg/rails,tgxworld/rails,elfassy/rails,lcreid/rails,pvalena/rails,iainbeeston/rails,kachick/rails,vassilevsky/rails,sergey-alekseev/rails,bradleypriest/rails,richseviora/rails,notapatch/rails,rafaelfranca/omg-rails,arjes/rails,betesh/rails,pvalena/rails,kirs/rails-1,mechanicles/rails,esparta/rails,bogdanvlviv/rails,travisofthenorth/rails,printercu/rails,Stellenticket/rails,matrinox/rails,mechanicles/rails,iainbeeston/rails,arunagw/rails,rossta/rails,Sen-Zhang/rails,Spin42/rails,mohitnatoo/rails,rails/rails,alecspopa/rails,palkan/rails,richseviora/rails,printercu/rails,untidy-hair/rails,kaspth/rails,georgeclaghorn/rails,Spin42/rails,tijwelch/rails,MSP-Greg/rails,samphilipd/rails,jeremy/rails,lcreid/rails,tijwelch/rails,odedniv/rails,arjes/rails,rafaelfranca/omg-rails,Vasfed/rails,deraru/rails,schuetzm/rails,aditya-kapoor/rails,jeremy/rails,Edouard-chin/rails,joonyou/rails,mijoharas/rails,starknx/rails,vipulnsward/rails,yalab/rails,illacceptanything/illacceptanything,tgxworld/rails,maicher/rails,alecspopa/rails,lcreid/rails,tgxworld/rails,MSP-Greg/rails,fabianoleittes/rails,illacceptanything/illacceptanything,palkan/rails,baerjam/rails,baerjam/rails,flanger001/rails,BlakeWilliams/rails,sealocal/rails,pschambacher/rails,riseshia/railsguides.kr,printercu/rails,assain/rails,bogdanvlviv/rails,gauravtiwari/rails,hanystudy/rails,kenta-s/rails,kenta-s/rails,baerjam/rails,EmmaB/rails-1,aditya-kapoor/rails,untidy-hair/rails,koic/rails,Spin42/rails,ttanimichi/rails,Sen-Zhang/rails,starknx/rails,mtsmfm/rails,felipecvo/rails,ttanimichi/rails,Vasfed/rails,mohitnatoo/rails,eileencodes/rails,Envek/rails,esparta/rails,palkan/rails,utilum/rails,BlakeWilliams/rails,yahonda/rails,gcourtemanche/rails,shioyama/rails,maicher/rails,illacceptanything/illacceptanything,mathieujobin/reduced-rails-for-travis,repinel/rails,marklocklear/rails,rossta/rails,deraru/rails,tijwelch/rails,vassilevsky/rails,yhirano55/rails,mijoharas/rails,Edouard-chin/rails,illacceptanything/illacceptanything,lucasmazza/rails,odedniv/rails,kamipo/rails,kmcphillips/rails,kachick/rails,baerjam/rails,BlakeWilliams/rails,sergey-alekseev/rails,joonyou/rails,mtsmfm/rails,bol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