cmd/pull: fix HTTP header usage.

Library/Homebrew/cmd/pull.rb

@@ -503,12 +503,12 @@ module Homebrew
         url = URI(bottle_info.url)
         puts "Verifying bottle: #{File.basename(url.path)}"
         http = Net::HTTP.new(url.host, url.port)
-        http.initialize_http_header "User-Agent" => HOMEBREW_USER_AGENT_RUBY
         http.use_ssl = true
         retry_count = 0
         http.start do
           while true do
             req = Net::HTTP::Head.new bottle_info.url
+            req.initialize_http_header "User-Agent" => HOMEBREW_USER_AGENT_RUBY
             res = http.request req
             if res.is_a?(Net::HTTPSuccess)
               break