code: public HttpRequest withHeader(String header, String value) {
  		if(Strings.isValid(header)) {
  			if(Strings.isValid(value)) {
  				headers.put(header,  value);
  			} else {
  				withoutHeader(header);
  			}			
  		}
  		return this;
  	}
docs: Sets the value of the given header, replacing whatever was already
  in there; if the value is null or empty, the header is dropped
  altogether.
  
  @param header
  the name of the header to set.
  @param value
  the new value for the header.
  @return
  the object itself, for method chaining.