def []=(key, value)
      h = self[key]      
      if h
        h.value = value
      else
        new_header = Redhead::Header.new(key, TO_RAW[key], value)
        self << new_header
      end
    end
def []=(key, value)
      h = self[key]      
      if h
        h.value = value
      else
        new_header = Redhead::Header.new(key, TO_RAW[key], value)
        self << new_header
      end
    end
If there is a header in the set with a key matching _key_, then set its value to _value_.
 If there is no header matching _key_, create a new header with the given key and value.