Enhance the String class with a XML escaped character version of to_s.

Methods
Public Instance methods
to_xs()

XML escaped version of to_s

     # File lib/builder/xchar.rb, line 107
107:   def to_xs
108:     unpack('U*').map {|n| n.xchr}.join # ASCII, UTF-8
109:   rescue
110:     unpack('C*').map {|n| n.xchr}.join # ISO-8859-1, WIN-1252
111:   end