Make sure synced folder types are always symbols

This will make sure that plugins that define synced folders after this
config object has been finalized won't have trouble because of
forgetting to use symbols instead of strings.

References https://github.com/fgrehm/vagrant-cachier/issues/76

plugins/kernel_v2/config/vm.rb

@@ -150,6 +150,8 @@ module VagrantPlugins
         options = options.dup
         options[:guestpath] = guestpath.to_s.gsub(/\/$/, '')
         options[:hostpath]  = hostpath
+        # Make sure the type is a symbol
+        options[:type] = options[:type].to_sym if options[:type]
 
         @__synced_folders[options[:guestpath]] = options
       end
@@ -324,9 +326,6 @@ module VagrantPlugins
             options[:type] = :nfs
           end
 
-          # Make sure the type is a symbol
-          options[:type] = options[:type].to_sym if options[:type]
-
           # Ignore NFS on Windows
           if options[:type] == :nfs && Vagrant::Util::Platform.windows?
             options.delete(:type)