# File lib/mail/header.rb, line 142
    def []=(name, value)
      name = dasherize(name)
      fn = name.downcase
      selected = select_field_for(fn)
      
      case
      # User wants to delete the field
      when !selected.blank? && value == nil
        fields.delete_if { |f| selected.include?(f) }
        
      # User wants to change the field
      when !selected.blank? && limited_field?(fn)
        selected.first.update(fn, value)
        
      # User wants to create the field
      else
        # Need to insert in correct order for trace fields
        self.fields << Field.new(name.to_s, value, charset)
      end
    end