Fix: ArrayPool<T>.Rent can return array longer than minimumLength

[0xfeeddeadbeef]

It should not be assumed that the returned array will be exactly the length requested.

For example, if you run this script using Windows PowerShell, eventually red messages will start appearing:

using assembly "C:\Users\UserName\.nuget\system.buffers\4.4.0\lib\netstandard2.0\System.Buffers.dll"

foreach ($i in 1..300000) {
  $a = [System.Buffers.ArrayPool[byte]]::Shared.Rent(16)
  if ($a.Length -ne 16) {
    Write-Host "Rented larger than minimumLength: $($a.Length)" -ForegroundColor Red
  }
}

[neuecc]

Thank you, this is a correct and important point!