code: public function defaultHeader(array $headers = [])
  	{
  		$this->defaultHeaders = array_merge($this->defaultHeaders, $headers);
  
  		return $this->defaultHeaders;
  	}
docs: Add one or more default headers
  
  also returns current default headers
  
  @param array $headers header(s) to be added
  
  @return array list of current default headers