ssl: allow underlying socket to not implement #sync

The value is used to determine whether SSLSocket should skip buffering
in OpenSSL::Buffering or not. Defaulting to true (no buffering) should
be a safe option.

lib/openssl/buffering.rb

@@ -64,7 +64,7 @@ def initialize(*)
     super
     @eof = false
     @rbuffer = Buffer.new
-    @sync = @io.sync
+    @sync = @io.respond_to?(:sync) ? @io.sync : true
   end
 
   #