Fix header extraction logic in Http class to account for buffer length

[joanhey]

Before always return the substr(), even when it's the end of the buffer.

Before:
$header = isset($buffer[$length]) ? substr($buffer, 0, $length) : $buffer;

Now:
$header = isset($buffer[$length + 1]) ? substr($buffer, 0, $length) : $buffer;

[joanhey]

Sorry, but I need this PR merged for the benchmark.
A bench that I don't like but it's the same, the final users only see the results.