🩹 Correctly split headers without leading whitespace

[RafaelKr]

Fixes #401.

According to https://datatracker.ietf.org/doc/html/rfc7230#section-3.2 headers may contain optional leading whitespace before the value. The previous implementation threw an error if there was no whitespace.

[QWp6t]

Thank you! 🙏

[RafaelKr]

@QWp6t No problem!

I built a little Benchmark: https://github.com/RafaelKr/php-header-split-bench
Usage and the output/results I got are documented in the README.md.

As you can see by just using explode the header values are actually modified because one leading space is added.
Usually (and according to RFC) this shouldn't be a problem, but it definitely modifies the current behavior:

{
    "X-With-Leading-Space": " Test: 123",
    "X-With-Leading-Space-And-Colon": " Test: 123",
    "X-Without-Leading-Space": "Test-Header",
    "Content-Type": " application\/x-www-form-urlencoded",
    "Cache-Control": " private, must-revalidate"
}

The same behavior can also be observed in real usage with Laravel/Acorn.

I think using preg_split wouldn't really impact performance but provide consistent output independent of leading whitespace.