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

Methods
Public Instance methods
to_xs(escape=true)

XML escaped version of to_s. When escape is set to false the CP1252 fix is still applied but utf-8 characters are not converted to character entities.

     # File lib/builder/xchar.rb, line 191
191:     def to_xs(escape=true)
192:       unpack('U*').map {|n| n.xchr(escape)}.join # ASCII, UTF-8
193:     rescue
194:       unpack('C*').map {|n| n.xchr}.join # ISO-8859-1, WIN-1252
195:     end