Allow setting default Resolv::DNS config in Resolv.new

Instead of just passing the use_ipv6 option, pass all options
given. By using a singular hash argument instead of keywords,
this avoids the issue when a user does:

```ruby
Resolv.new([Resolv::DNS.new], use_ipv6: false)
```

Which would have resulted in the use_ipv6 option being ignored.

This is backwards compatible, because there has not yet been
a release with the use_ipv6 keyword supported.

Author: jeremyevans

lib/resolv.rb

@@ -83,9 +83,18 @@ def self.each_name(address, &proc)
 
   ##
   # Creates a new Resolv using +resolvers+.
-
-  def initialize(resolvers=nil, use_ipv6: nil)
-    @resolvers = resolvers || [Hosts.new, DNS.new(DNS::Config.default_config_hash.merge(use_ipv6: use_ipv6))]
+  #
+  # If +resolvers+ is not given, a hash, or +nil+, uses a Hosts resolver and
+  # and a DNS resolver.  If +resolvers+ is a hash, uses the hash as
+  # configuration for the DNS resolver.
+
+  def initialize(resolvers=nil)
+    @resolvers = case resolvers
+    when Hash, nil
+      [Hosts.new, DNS.new(DNS::Config.default_config_hash.merge(resolvers || {}))]
+    else
+      resolvers
+    end
   end
 
   ##