fix: add globalheader to client#getNodesInfo()

[AkisAya]

i use go-elasticsearch to visit elasticsearch servers, and due to the restrictions of my company, i have to add a custom header to the request. go-elasticsearch config and underlying transport client config provides a global header field to do this thing, like the bode below

c, err := es8.NewTypedClient(es8.Config{
	Addresses:            url,
	Username:             name,
	Password:             pwd,
	DiscoverNodesOnStart: true,
	Header:               globalHeader,
})

it works when i just perform a search, because the underlying transport client set the global header before performing a request like this

func (c *Client) Perform(req *http.Request) (*http.Response, error) {
	var (
		res *http.Response
		err error
	)
        ...
	// Update request
	c.setReqUserAgent(req)
	c.setReqGlobalHeader(req)
        ...
}

c.setReqGlobalHeader(req) is used to set the global header.

but when it comes to discoverNodes(), the global header is not set, leading to discoverNodes failure in my case

func (c *Client) getNodesInfo() ([]nodeInfo, error) {
	var (
		out    []nodeInfo
		scheme = c.urls[0].Scheme
	)

	req, err := http.NewRequest("GET", "/_nodes/http", nil)
	...

	c.setReqURL(conn.URL, req)
	c.setReqAuth(conn.URL, req)
	c.setReqUserAgent(req)

	res, err := c.transport.RoundTrip(req)
        ...
}

I think this is a mistake, and this pr add c.setReqGlobalHeader(req) to getNodesInfo() to set global header

[AkisAya]

Hi @Anaethelion , would you mind reviewing this PR whenever you are available?