public function addClientHeader(string $headerName, $headerValue): self
    {
        $this->clientConfig['headers'][$headerName] = $headerValue;
        return $this;
    }
public function addClientHeader(string $headerName, $headerValue): self
    {
        $this->clientConfig['headers'][$headerName] = $headerValue;
        return $this;
    }
public function addClientHeader ( string $ headerName , $ headerValue ) : self { $ this -> clientConfig [ 'headers' ] [ $ headerName ] = $ headerValue ; return $ this ; }
add a client request header
@param string $headerName
@param mixed $headerValue
@return self
add a client request header