⚡ Slightly improve getRequestIP performances

[homersimpsons]

Previous implementations shortcommings:

(_header || "")?.split(",").shift()?.trim();
         |     |           |       \- Useless as we have a string before `split`, so at least 1 string element in the array
         |     |           \- Useless as we can just get the first character `[0]`
         |     \- Useless as we cannot have undefined before
         \- Useless as we can just skip the whole computation if the header is falsy

• Remove header fallback to empty string
• Remove useless undefined check
• Uses [0] instead of shifting

Note that I also tried to play with substring / slice and use split(',', 2) but that didn't seem to improve the performances.

Resolves #1196

perf.link reports that the new implementation is twice faster on a micro-benchmark

[codecov]

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✅ All modified and coverable lines are covered by tests.

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[homersimpsons]

If we want to code-golf this we can use &&, do you prefer that I write it like this?

Suggested change

	if (_header) {
	const xForwardedFor = _header.split(",")[0].trim();
	if (xForwardedFor) {
	return xForwardedFor;
	}
	}
	const xForwardedFor = _header && _header.split(",")[0].trim();
	if (xForwardedFor) {
	return xForwardedFor;
	}

(this may have a slight, but not noticeable impact, as in case _header is falsy it will have to be evaluated as falsy twice)