Fix for Text::vCard::Node->types()

Previously types() was returning all the keys from Text::vCard::Node->{params}.  However that params hash contains more than just types.  It contains all possible params like 1 => 'pref' for example which is not a type.  Here is an example of the params hash dumped out on the command line:

        params         {
          1      "pref",
         work   "type"
       },

This pull request fixes types() so it only returns types.