code: public HttpRequest withParameter(HttpParameter parameter) {
  		if(parameter != null) {
  			parameters.add(parameter);
  		} else {
  			withoutParameter(parameter);			
  		}
  		return this;
  	}
docs: Sets the value of the given parameter, replacing whatever was
  already in there; if the value is null or empty, the parameter
  is dropped altogether.
  
  @param parameter
  the name of the parameter to set.
  @param value
  the new value for the parameter.
  @return
  the object itself, for method chaining.