Only split and examine VARY header in Deflater when present - #2463

When the VARY response header isn't set, there is no need to parse it
and examine it's content.

This improves the Deflater middleware by 20% when the header isn't set:

```ruby

require 'benchmark/ips'
require 'rack'

module Rack
  class Deflater2 < Deflater
    VARY = 'vary'

    def call(env)
      status, headers, body = response = @app.call(env)

      unless should_deflate?(env, status, headers, body)
        return response
      end

      request = Request.new(env)

      encoding = Utils.select_best_encoding(%w(gzip identity),
                                            request.accept_encoding)

      # Set the Vary HTTP header.
      if headers.has_key?(VARY)
        vary = headers[VARY].to_s.split(",").map(&:strip)
        unless vary.include?("*") || vary.any?{|v| v.downcase == 'accept-encoding'}
          headers[VARY] = vary.push("Accept-Encoding").join(",")
        end
      else
        headers[VARY] = "Accept-Encoding"
      end

      case encoding
      when "gzip"
        headers['content-encoding'] = "gzip"
        headers.delete(CONTENT_LENGTH)
        mtime = headers["last-modified"]
        mtime = Time.httpdate(mtime).to_i if mtime
        response[2] = GzipStream.new(body, mtime, @sync)
        response
      when "identity"
        response
      else # when nil
        # Only possible encoding values here are 'gzip', 'identity', and nil
        message = "An acceptable encoding for the requested resource #{request.fullpath} could not be found."
        bp = Rack::BodyProxy.new([message]) { body.close if body.respond_to?(:close) }
        [406, { CONTENT_TYPE => "text/plain", CONTENT_LENGTH => message.length.to_s }, bp]
      end
    end

  end
end

app = lambda do |env|
  [200, {}, []]
end

deflater = Rack::Deflater.new(app, {})
deflater2 = Rack::Deflater2.new(app, {})
request = Rack::MockRequest.env_for('', {})

Benchmark.ips do |x|
  x.config(warmup: 2, time: 5)

  x.report("main") do
    deflater.call(request)
  end

  x.report("branch") do
    deflater2.call(request)
  end
  x.compare!
end
```

```
ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [arm64-darwin24]
Warming up --------------------------------------
                main    94.625k i/100ms
              branch   115.769k i/100ms
Calculating -------------------------------------
                main    950.471k (± 1.0%) i/s    (1.05 μs/i) -      4.826M in   5.077838s
              branch      1.173M (± 1.3%) i/s  (852.46 ns/i) -      5.904M in   5.034024s

Comparison:
              branch:  1173071.0 i/s
                main:   950471.1 i/s - 1.23x  slower
```