Follow-up to 83a6ac3f.

This allows `HTTP_HOST` to be omitted as before, and reduces the number
of object allocations per request.

Benchmark:

```ruby
 # frozen_string_literal: true
require "benchmark/memory"

HOST = "example.com:80"
BEFORE_REGEXP = /\A(?<host>[a-z0-9.-]+|\[[a-f0-9]*:[a-f0-9.:]+\])(:\d+)?\z/
AFTER_REGEXP = /(?:\A|,[ ]?)([a-z0-9.-]+|\[[a-f0-9]*:[a-f0-9.:]+\])(?::\d+)?\z/i

Benchmark.memory do |x|
  x.report("BEFORE (non-nil X-Forwarded-Host)") do
    origin_host = BEFORE_REGEXP.match(HOST.to_s.downcase)[:host]
    forwarded_host = BEFORE_REGEXP.match(HOST.to_s.split(/,\s?/).last)[:host]
  end

  x.report("BEFORE (nil X-Forwarded-Host)") do
    origin_host = BEFORE_REGEXP.match(HOST.to_s.downcase)[:host]
    forwarded_host = BEFORE_REGEXP.match(nil.to_s.split(/,\s?/).last)
  end

  x.report("AFTER (non-nil X-Forwarded-Host)") do
    origin_host = HOST&.slice(AFTER_REGEXP, 1) || ""
    forwarded_host = HOST&.slice(AFTER_REGEXP, 1) || ""
  end

  x.report("AFTER (nil X-Forwarded-Host)") do
    origin_host = HOST&.slice(AFTER_REGEXP, 1) || ""
    forwarded_host = nil&.slice(AFTER_REGEXP, 1) || ""
  end
end
```

Results:

```
BEFORE (non-nil X-Forwarded-Host)
                       616.000  memsize (   208.000  retained)
                         9.000  objects (     2.000  retained)
                         2.000  strings (     1.000  retained)
BEFORE (nil X-Forwarded-Host)
                       328.000  memsize (     0.000  retained)
                         5.000  objects (     0.000  retained)
                         2.000  strings (     0.000  retained)
AFTER (non-nil X-Forwarded-Host)
                       248.000  memsize (   168.000  retained)
                         3.000  objects (     1.000  retained)
                         1.000  strings (     0.000  retained)
AFTER (nil X-Forwarded-Host)
                        40.000  memsize (     0.000  retained)
                         1.000  objects (     0.000  retained)
                         1.000  strings (     0.000  retained)
```

[CVE-2021-22942]