Do not modify config returned by Resolv::DNS::Config (#306)

* Do not modify config returned by Resolv::DNS::Config

* Update lib/valid_email2/dns.rb

---------

Co-authored-by: Peter Schröder <[email protected]>

Author: mpokrywka

lib/valid_email2/dns.rb

@@ -64,7 +64,8 @@ def fetch(domain, type)
 
     def resolv_config
       config = Resolv::DNS::Config.default_config_hash
-      config[:nameserver] = @dns_nameserver if @dns_nameserver
+      # REM: resolv gem 0.6.1 on linux can return frozen hash
+      config = config.merge(nameserver: @dns_nameserver) if @dns_nameserver
       config
     end
   end