fix: Remove the default connection close header

Instead, we rely on the underlying http implementation in Node.js
to handle this, as per the documentation at
https://nodejs.org/api/http.html#new-agentoptions

This fixes node-fetch#1735 and likely replaces node-fetch#1473

The original change introducing this provided no clear motivation
for the override, and the implementation has since been changed to
disable this header when an agent is provided, so I think there
is sufficient evidence that removing this is the correct behaviour.
node-fetch@af21ae6
node-fetch@7f68577

This commit is backported to the v2 branch from node-fetch#1736 against v3.

README.md

@@ -387,7 +387,6 @@ Header              | Value
 ------------------- | --------------------------------------------------------
 `Accept-Encoding`   | `gzip,deflate` _(when `options.compress === true`)_
 `Accept`            | `*/*`
-`Connection`        | `close` _(when no `options.agent` is present)_
 `Content-Length`    | _(automatically calculated, if possible)_
 `Transfer-Encoding` | `chunked` _(when `req.body` is a stream)_
 `User-Agent`        | `node-fetch/1.0 (+https://github.com/bitinn/node-fetch)`
@@ -404,6 +403,8 @@ The `agent` option allows you to specify networking related options which are ou
 
 See [`http.Agent`](https://nodejs.org/api/http.html#http_new_agent_options) for more information.
 
+If no agent is specified, the default agent provided by Node.js is used. Note that [this changed in Node.js 19](https://github.com/nodejs/node/blob/4267b92604ad78584244488e7f7508a690cb80d0/lib/_http_agent.js#L564) to have `keepalive` true by default. If you wish to enable `keepalive` in an earlier version of Node.js, you can override the agent as per the following code sample. 
+
 In addition, the `agent` option accepts a function that returns `http`(s)`.Agent` instance given current [URL](https://nodejs.org/api/url.html), this is useful during a redirection chain across HTTP and HTTPS protocol.
 
 ```js

src/request.js

@@ -258,10 +258,6 @@ export function getNodeRequestOptions(request) {
 		agent = agent(parsedURL);
 	}
 
-	if (!headers.has('Connection') && !agent) {
-		headers.set('Connection', 'close');
-	}
-
 	// HTTP-network fetch step 4.2
 	// chunked encoding is handled by Node.js