Enhance the Fixnum class with a XML escaped character conversion.

Methods
Constants
XChar = Builder::XChar if ! defined?(XChar)
Public Instance methods
xchr(escape=true)

XML escaped version of chr. 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 171
171:     def xchr(escape=true)
172:       n = XChar::CP1252[self] || self
173:       case n when *XChar::VALID
174:         XChar::PREDEFINED[n] or 
175:           (n<128 ? n.chr : (escape ? "&##{n};" : [n].pack('U*')))
176:       else
177:         Builder::XChar::REPLACEMENT_CHAR
178:       end
179:     end