Fix keepAlive for Node.js 19+

[parfeon]

fix(node-fetch): keepAlive for Node.js 19+

Fix issue because of which node-fetch used default agent, which after Node.js 19+ has keepAlive enabled by default.

[parfeon]

In this section of node-fetch docs written the following:

If no agent is specified, the default agent provided by Node.js is used. Note that this changed in Node.js 19 to have keepalive true by default.

In the code, I added this guard:

// Don't configure any agents if keep alive not requested.
if (!this.keepAlive && !this.proxyConfiguration) return undefined;

and with mentioned default behavior, it means that if user will set keepAlive to false we won't set agent and default will be used where in Node.js 19+ keepAlive is set to true by default.